feat(session): reset running session start time to now from list

Adds a confirm-gated button on running sessions in the session list that
sets timestamp_start to now (issue #33). The htmx path returns HX-Refresh;
ButtonGroup gains optional hx_confirm/hx_swap keys.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/test_session_reset_e2e.py

@@ -0,0 +1,46 @@
+"""Browser test for the session-list "Reset start to now" button (issue #33).
+
+Drives the real session list against pytest-django's ``live_server``: clicks the
+reset button on a running session, accepts the confirm dialog, and asserts the
+row's start time is updated in place via htmx.
+"""
+
+import datetime as dt
+
+import pytest
+from django.urls import reverse
+from playwright.sync_api import Page, expect
+
+from games.models import Game, Platform, Session
+
+
+@pytest.fixture
+def authenticated_page(live_server, page: Page, django_user_model) -> Page:
+    django_user_model.objects.create_user(username="tester", password="secret123")
+    page.goto(f"{live_server.url}{reverse('login')}")
+    page.fill('input[name="username"]', "tester")
+    page.fill('input[name="password"]', "secret123")
+    page.click('button:has-text("Login")')
+    page.wait_for_url(f"{live_server.url}/tracker**")
+    return page
+
+
+def test_reset_session_start_to_now(authenticated_page: Page, live_server):
+    page = authenticated_page
+    platform = Platform.objects.create(name="PC", icon="pc", group="PC")
+    game = Game.objects.create(name="Reset Game", platform=platform)
+    session = Session.objects.create(
+        game=game,
+        timestamp_start=dt.datetime(2020, 1, 1, 10, 0, tzinfo=dt.timezone.utc),
+    )
+
+    page.goto(f"{live_server.url}{reverse('games:list_sessions')}")
+
+    row = page.locator(f"#session-row-{session.id}")
+    expect(row).to_contain_text("2020")
+
+    page.on("dialog", lambda dialog: dialog.accept())
+    row.locator('button[title="Reset start to now"]').click()
+
+    # htmx swaps the row in place; the old 2020 start time is gone.
+    expect(row).not_to_contain_text("2020")