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>

ts/elements/selection-fields.ts

@@ -2,7 +2,7 @@ import { readSelectionFieldsProps, SelectionFieldsProps } from "../generated/pro
 
 /**
  * Renders one form field per selected item of a source SearchSelect (matched by
- * its data-name). Reacts to the SearchSelect's "search-select:change" event and
+ * its name attribute). Reacts to the SearchSelect's "search-select:change" event and
  * to its own "active" attribute. Typed values are preserved (keyed by item id)
  * across selection changes and active toggling.
  */
@@ -24,7 +24,7 @@ class SelectionFieldsElement extends HTMLElement {
   connectedCallback(): void {
     this.props = readSelectionFieldsProps(this);
     this.source = document.querySelector<HTMLElement>(
-      `[data-search-select][data-name="${this.props.source}"]`,
+      `search-select[name="${this.props.source}"]`,
     );
     document.addEventListener("search-select:change", this.onSourceChange);
     this.render();