feat(document-viewer): add PDF text search with highlight overlay

[mx-kshitij]

Pull request type

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Description

Adds in-document text search to the PDF viewer in document-viewer-web.

What changed:

• PDFViewer.tsx — A search toggle button (magnifier icon) is added to the control bar. When activated, a secondary search bar appears below the controls with:
  • A debounced text input (300 ms) to avoid searching on every keystroke.
  • A live "N of M" / "No results" / "Searching…" counter (aria-live).
  • Previous / Next match navigation buttons that also navigate across pages.
  • Escape key closes the search bar and clears the query.
  • Auto-focus on the input when the bar opens.
• usePDFSearch.ts (new) — Custom hook that iterates all pages of the loaded PDFDocumentProxy, collects all case-insensitive substring matches as SearchMatch records (page, item index, char offsets), and exposes goToNextMatch / goToPrevMatch with automatic page navigation.
• usePDFHighlightPositions.ts (new) — Custom hook that converts SearchMatch records on the current page into absolutely-positioned HighlightRect objects. Uses page.getViewport() + a canvas context for proportional glyph-width measurement scaled to the actual rendered item width. Async effects are guarded with an active flag to prevent state updates after unmount/query change.
• BaseViewer.tsx — Added optional SecondaryControl prop slot rendered between the toolbar and the page content.
• documentViewer.scss — Styles for .widget-document-viewer-search-bar, .widget-document-viewer-highlight-layer, and .widget-document-viewer-highlight (yellow for all matches, orange + outline for the current match). Uses Atlas UI tokens (--spacing-*, --border-color-default, etc.).
• documentViewerIcons.scss — Added Search icon (\e905) to the DocViewer icon font map; broadened font-family scope so search-bar nav buttons also pick up the icon font.
• DocViewer.woff2 — Updated to include the new search glyph.

Why:
Users viewing multi-page PDFs had no way to locate specific text without scrolling manually. This brings standard Ctrl+F-style search behaviour to the widget, matching expectations from native PDF viewers.

Note: A semver bump (1.2.0 → 1.3.0) and a CHANGELOG.md entry are needed before merging. No XML/property changes; no docs PR required.

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What should be covered while testing?

1. Open a multi-page PDF in the Document Viewer widget.
2. Click the magnifier (Search) button in the toolbar — the search bar should appear below the controls with focus on the input.
3. Type a term that appears in the document:
   • Highlights (yellow) should appear over all matches on the current page.
   • The counter shows "1 of N".
4. Click Next / Previous to cycle through matches — the current match turns orange with an outline; the page changes automatically when crossing a page boundary.
5. Type a term with no matches — counter shows "No results", no highlights.
6. Press Escape — search bar closes and all highlights clear.
7. Load a different document — search state resets completely.
8. Verify the screen reader announces match count changes (the count span uses aria-live="polite").
9. Verify the Search toggle button reflects aria-pressed="true" when open.