Convert onSwap widgets to custom elements (issue #18)

Replaces the four onSwap-based widgets with TypeScript custom elements
following the pattern from PR #16. Each widget gets a class extending
HTMLElement with connectedCallback/disconnectedCallback, typed props via
register_element + gen_element_types codegen, and lives in ts/elements/.

- range-slider: RangeSliderElement; Python uses _RangeSlider builder
- date-range-picker: DateRangePickerElement; Python uses _DateRangePicker builder
- search-select: SearchSelectElement; Python uses _SearchSelect builder;
  data-* attrs become plain attrs (data-name -> name, data-search-url -> search-url, etc.)
- filter-bar: FilterBarElement; props carry preset URLs; onclick/onsubmit
  attrs replaced with data-filter-bar-* sentinel attrs; all window.* globals removed

Deletes ts/range_slider.ts, ts/search_select.ts, ts/date_range_picker.ts,
ts/filter_bar.ts. Updates all tests and e2e pages to use the new element
selectors and script paths (dist/elements/<tag>.js).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_date_range_picker.py

@@ -145,7 +145,7 @@ class DateRangePickerTest(SimpleTestCase):
                 max_value="2024-12-31",
             )
         )
-        self.assertIn("data-date-range-picker", html)
+        self.assertIn("<date-range-picker", html)
         self.assertIn('data-input-name-prefix="filter-date-purchased"', html)
         self.assertIn("data-date-range-field", html)
         self.assertIn("data-date-range-calendar", html)
@@ -166,7 +166,7 @@ class PurchaseFilterBarDateRangePickerTest(TestCase):
 
     def test_purchased_uses_date_range_picker(self):
         html = self.render()
-        self.assertIn("data-date-range-picker", html)
+        self.assertIn("<date-range-picker", html)
         self.assertIn('data-input-name-prefix="filter-date-purchased"', html)
         # The hidden ISO inputs keep the names filter_bar.js serializes.
         self.assertIn('name="filter-date-purchased-min"', html)