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_rendered_pages.py

@@ -63,9 +63,9 @@ class RenderedPagesTest(TestCase):
         """The games list view passes no scripts= argument; the filter bar's
         components declare their JS and Page() collects it."""
         html = self.get("games:list_games").content.decode()
-        self.assertIn("js/dist/filter_bar.js", html)
-        self.assertIn("js/dist/search_select.js", html)
-        self.assertIn("js/dist/range_slider.js", html)
+        self.assertIn("js/dist/elements/filter-bar.js", html)
+        self.assertIn("js/dist/elements/search-select.js", html)
+        self.assertIn("js/dist/elements/range-slider.js", html)
 
     def test_stats_page_auto_loads_datepicker(self):
         """YearPicker declares the datepicker UMD bundle as media; the stats