feat(files): render Files tab diffs with @pierre/diffs#4112

Summary

• Replace the hand-written unified-diff parser/renderer in the session Files tab with @pierre/diffs, gaining real Shiki syntax highlighting and built-in row virtualization.
• SessionFilesView parses each file's diff via getSingularPatch and renders it with @pierre/diffs/react's FileDiff, wrapped in its Virtualizer (replaces the @tanstack/react-virtual row window).
• Line/file annotations map onto FileDiff's lineAnnotations/renderAnnotation and a custom renderGutterUtility button; selection → instruction now uses @pierre/diffs' own line-selection API (enableLineSelection, onLineSelectionChange, selectedLines) instead of native browser text selection, still surfaced through the existing right-click DiffSelectionMenu.
• New src/renderer/lib/diff-lines.ts adapts @pierre/diffs' parsed FileDiffMetadata into the flat, line-numbered lookup the annotation and selection features need.
• Split/unified toggle, new-file rendering, and live SSE-driven updates are unchanged; theme is wired to pierre-dark/pierre-light via the app's resolved light/dark mode.
• Removes diff-parser.ts, its worker, and useParsedDiff.ts.
• No backend/API contract changes.

Demo

Test plan

• npm run typecheck
• Full test suite (documented one pre-existing, unrelated packages/product-ui gap)
• Manual E2E pass against a real local daemon: Files tab open, syntax highlighting, file-level annotation send (real backend round-trip), search filter, split/unified toggle, maximize/restore