feat(app): Cmd+F find in Preview PDFs (S1)

[jacktrnr]

Summary

S1 of Cmd+F find in Preview PDFs — the vertical slice. Cmd+F on a focused PDF pane opens a find pill, highlights every match, and steps through them.

• Find core (pdf-find-core.ts): walks the pdf.js text layer into a normalized searchable string + DOM offset map. Handles hyphen-line-break joins, whitespace collapse, NFKC (ligatures), per-page tagging.
• Painter (pdf-find-highlight.ts): CSS Custom Highlight API only — never mutates the DOM, so selection/copy/links are untouched.
• Controller (pdf-find-controller.ts): per-pane state, lazy index, debounced query, pane-scoped scroll (never scrollIntoView).
• Pill (pdf-find-pill.tsx): floating find bar in the preview controls; exempt from idle auto-hide.
• Routing + guard (pdf-find-guard.ts, session-preview-tab.tsx, entry.tsx): focus-based Cmd+F routing; a boot-time guard preventDefaults the key inside a PDF pane so upstream file-find (which captures Cmd+F whenever a diff/tool card is mounted) can't hijack it — stopPropagation is never called, so the command registry still fires.

Test plan

• Unit: 73/73 pass (bun test src/components/session/ in the app package), incl. 28 new find-core tests.
• Typecheck: clean (tsgo).
• Materialize + drift gate: PASS.
• Verified live: rebuilt locally, opened a PDF in Preview, Cmd+F → pill opens, matches highlight, Enter steps, Esc closes.
• Pending: Playwright e2e (needs dev server) — tracked as a follow-up.

Part of the "Find in Preview" design (parent issue draft in scratchpad). S2–S4 (options, recompile survival, markdown) come next.

[coderabbitai]

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