feat(date-range-filter): keyboard navigation between date parts

Closes #64. The segmented date-range field now responds to arrow keys:

- Left/Right move focus between DD/MM/YYYY parts, crossing the min→max
  separator; focus clamps at the first/last part (no wrap).
- Up/Down increment/decrement the focused part, clamped to its valid
  range (day 1-31, month 1-12, year 1-9999). An empty part seeds to 01
  for day/month and the current year for year on the first press.

Arrows with modifiers (Ctrl/Alt/Meta) still fall through to native
behavior. Adds e2e coverage for focus walking, boundary clamping, value
stepping, hidden-ISO commit, and modifier passthrough.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

e2e/test_date_range_picker_e2e.py

@@ -18,6 +18,7 @@ import urllib.parse
 import pytest
 from django.http import HttpResponse
 from django.test import override_settings
+from playwright.sync_api import expect
 
 from common.components import PurchaseFilterBar
 from django.urls import path
@@ -324,3 +325,136 @@ def test_prefilled_picker_round_trips_unchanged(live_server, page):
         "value2": "2024-09-20",
         "modifier": "BETWEEN",
     }
+
+
+# ── Keyboard navigation ─────────────────────────────────────────────────────
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_right_moves_focus_across_segments(live_server, page):
+    """ArrowRight walks DD → MM → YYYY and crosses the min→max separator."""
+    page.goto(live_server.url + "/test-date-range-picker/")
+    _segment(page, "min", "day").click()
+    page.keyboard.press("ArrowRight")
+    expect(_segment(page, "min", "month")).to_be_focused()
+    page.keyboard.press("ArrowRight")
+    expect(_segment(page, "min", "year")).to_be_focused()
+    page.keyboard.press("ArrowRight")
+    expect(_segment(page, "max", "day")).to_be_focused()
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_left_moves_focus_backwards(live_server, page):
+    """ArrowLeft from the max side's first part lands on the min side's last."""
+    page.goto(live_server.url + "/test-date-range-picker/")
+    _segment(page, "max", "day").click()
+    page.keyboard.press("ArrowLeft")
+    expect(_segment(page, "min", "year")).to_be_focused()
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_left_clamps_at_first_segment(live_server, page):
+    page.goto(live_server.url + "/test-date-range-picker/")
+    _segment(page, "min", "day").click()
+    page.keyboard.press("ArrowLeft")
+    expect(_segment(page, "min", "day")).to_be_focused()
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_right_clamps_at_last_segment(live_server, page):
+    page.goto(live_server.url + "/test-date-range-picker/")
+    _segment(page, "max", "year").click()
+    page.keyboard.press("ArrowRight")
+    expect(_segment(page, "max", "year")).to_be_focused()
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_up_on_empty_day_seeds_value(live_server, page):
+    """First ArrowUp on an empty part lands on its seed (day → 01)."""
+    page.goto(live_server.url + "/test-date-range-picker/")
+    day_segment = _segment(page, "min", "day")
+    day_segment.click()
+    page.keyboard.press("ArrowUp")
+    assert day_segment.input_value() == "01"
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_up_on_empty_year_uses_current_year(live_server, page):
+    page.goto(live_server.url + "/test-date-range-picker/")
+    year_segment = _segment(page, "min", "year")
+    year_segment.click()
+    page.keyboard.press("ArrowUp")
+    assert year_segment.input_value() == str(datetime.date.today().year)
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_up_down_increments_and_clamps_day(live_server, page):
+    """Up increments and clamps at 31; Down clamps at 01 (no wrap)."""
+    page.goto(live_server.url + "/test-date-range-picker/")
+    day_segment = _segment(page, "min", "day")
+    # Typing a full 2-digit part auto-advances, so re-focus before arrowing.
+    day_segment.click()
+    page.keyboard.type("28")
+    day_segment.click()
+    for _ in range(4):
+        page.keyboard.press("ArrowUp")
+    assert day_segment.input_value() == "31"
+    page.keyboard.press("ArrowDown")
+    assert day_segment.input_value() == "30"
+    day_segment.click()
+    page.keyboard.type("01")
+    day_segment.click()
+    page.keyboard.press("ArrowDown")
+    assert day_segment.input_value() == "01"
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_up_down_clamps_month(live_server, page):
+    page.goto(live_server.url + "/test-date-range-picker/")
+    month_segment = _segment(page, "min", "month")
+    month_segment.click()
+    page.keyboard.type("12")
+    month_segment.click()
+    page.keyboard.press("ArrowUp")
+    assert month_segment.input_value() == "12"
+    month_segment.click()
+    page.keyboard.type("01")
+    month_segment.click()
+    page.keyboard.press("ArrowDown")
+    assert month_segment.input_value() == "01"
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_arrow_value_change_commits_complete_date(live_server, page):
+    """Stepping the last empty part to a valid value commits the hidden ISO."""
+    page.goto(live_server.url + "/test-date-range-picker/")
+    _segment(page, "min", "month").click()
+    page.keyboard.type("032024")  # month=03 → year=2024, day left empty
+    day_segment = _segment(page, "min", "day")
+    day_segment.click()
+    page.keyboard.press("ArrowUp")  # day → 01
+    assert page.locator(HIDDEN_MIN).input_value() == "2024-03-01"
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="e2e.test_date_range_picker_e2e")
+def test_ctrl_arrow_does_not_change_value(live_server, page):
+    """Ctrl+Arrow falls through to native — the segment value is untouched."""
+    page.goto(live_server.url + "/test-date-range-picker/")
+    day_segment = _segment(page, "min", "day")
+    day_segment.click()
+    page.keyboard.type("15")
+    day_segment.click()
+    page.keyboard.down("Control")
+    page.keyboard.press("ArrowUp")
+    page.keyboard.up("Control")
+    assert day_segment.input_value() == "15"

ts/elements/date-range-picker.ts

@@ -137,6 +137,23 @@ function segmentBuffer(segment: HTMLInputElement): string {
   return segment.dataset.typedDigits || "";
 }
 
+// The numeric bounds of a date part plus the value an empty part jumps to on
+// the first ArrowUp/ArrowDown (day/month start at 01, year at the current year
+// rather than 0001).
+interface PartRange {
+  min: number;
+  max: number;
+  empty: number;
+}
+
+function partRange(datePart: string): PartRange {
+  if (datePart === "month") return { min: 1, max: 12, empty: 1 };
+  if (datePart === "year") {
+    return { min: 1, max: 9999, empty: new Date().getFullYear() };
+  }
+  return { min: 1, max: 31, empty: 1 }; // day
+}
+
 function setSegmentBuffer(segment: HTMLInputElement, buffer: string): void {
   segment.dataset.typedDigits = buffer;
   if (buffer === "") {
@@ -229,6 +246,36 @@ function initField(picker: HTMLElement, calendarState: CalendarState): void {
         return;
       }
       if (event.ctrlKey || event.metaKey || event.altKey) return;
+      // Arrow keys move between parts (Left/Right) or step the focused part's
+      // value (Up/Down); handled before the digit-only path below. Out-of-range
+      // index clamps (no wrap); Up/Down clamp at the part's range ends.
+      if (event.key === "ArrowLeft" || event.key === "ArrowRight") {
+        event.preventDefault();
+        const step = event.key === "ArrowRight" ? 1 : -1;
+        const target = segments[segmentIndex + step];
+        if (target) {
+          target.focus();
+          target.select();
+        }
+        return;
+      }
+      if (event.key === "ArrowUp" || event.key === "ArrowDown") {
+        event.preventDefault();
+        const range = partRange(segment.dataset.datePart ?? "");
+        const width = parseInt(segment.getAttribute("maxlength") ?? "", 10);
+        const buffer = segmentBuffer(segment);
+        let next: number;
+        if (buffer === "") {
+          next = range.empty;
+        } else {
+          next = parseInt(buffer, 10) + (event.key === "ArrowUp" ? 1 : -1);
+        }
+        if (next < range.min) next = range.min;
+        if (next > range.max) next = range.max;
+        setSegmentBuffer(segment, padNumber(next, width));
+        syncHiddenFromSegments(picker, segment.dataset.dateSide ?? "");
+        return;
+      }
       event.preventDefault();
       if (!/^[0-9]$/.test(event.key)) return; // only numbers can be typed
       const maximumLength = parseInt(segment.getAttribute("maxlength") ?? "", 10);