Convert search_select.js to TypeScript (issue #17)

- Add ts/search_select.ts: typed port of the SearchSelect/FilterSelect widget.
  Exports SearchSelectOption / SearchSelectChangeDetail as the single source of
  truth for the "search-select:change" event contract
- add_purchase.ts now imports those types via `import type` (no runtime
  coupling), instead of redefining them locally
- Declare window.readSearchSelect in ts/globals.d.ts
- Point the SearchSelect component Media and every view/e2e/test reference at
  the compiled dist/search_select.js
- Update doc comments in common/components/search_select.py to name the TS source

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

e2e/test_date_range_picker_e2e.py

@@ -30,7 +30,7 @@ def _bar_page(filter_json: str = "") -> str:
     <title>Date range picker E2E</title>
     <script src="/static/js/htmx.min.js"></script>
     <script src="/static/js/range_slider.js" type="module"></script>
-    <script src="/static/js/search_select.js" type="module"></script>
+    <script src="/static/js/dist/search_select.js" type="module"></script>
     <script src="/static/js/date_range_picker.js" defer></script>
     <script src="/static/js/filter_bar.js" type="module"></script>
 </head>