Use a native <dialog> for the search modal

[benbalter]

What

The search modal was a div role="dialog" with a hand-rolled Tab trap. It's now a native <dialog> opened with showModal(), following the pattern ShortcutsModal already uses.

• Focus: the browser handles containment, inertness, and ::backdrop. The manual trapFocus handler and its focusable-selector list are gone.
• Closing: a single close event handler resets the input, results, and scroll lock, and returns focus to the trigger. It runs however the dialog closes: Esc, ⌘K, the close button, a backdrop click, or a result click.
• Esc: the explicit Esc handler stays, because a search input holding a query can use up the first Esc to clear itself.
• Backdrop clicks: detected with the same bounding-box check the shortcuts dialog uses.
• Styling: the dialog is now the panel itself. The fade and scale animation uses @starting-style plus allow-discrete, and falls back to no animation where unsupported. display: flex is scoped to [open] so it can't override the UA's hidden state. Light and dark tokens are unchanged.
• Live region: aria-live is off #search-results, which re-announced the whole list on every keystroke. A visually hidden role="status" element now announces only the count ("8 results", "No results").
• shortcuts.ts checks dialog.open instead of hidden.
• Rendering and navigation helpers are exported and covered by a new src/scripts/search-modal.test.ts, which tests escaping, title-as-text, arrow and j/k navigation, and count labels.

Verification

• npm run check: 0 errors
• npx vitest run src/scripts/search-modal.test.ts: 13 passed
• npx eslint on changed TS files: clean
• SKIP_PDF=1 npm run build: OK
• Playwright (chromium) search-shortcuts, csp, accessibility-astro against astro preview: 71 passed
• Checked by hand in Chrome: the trigger opens the dialog and focuses the input, the status reads "8 results", a backdrop click closes it and returns focus to the trigger, and Esc closes it and clears state. Light and dark screenshots look right.

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