[R1-02] Verify and finish PDF highlights and Annotations workflow

[dkritarth]

Outcome

Users can create durable highlights from actual PDF text, find them in Annotations, jump back, and delete them reliably.

Dependencies

Blocked by #29. Existing PR #22 is stacked on PR #21 and must remain ordered. PR #22 implements highlight behavior only; Add to chat and Explain belong to later issue.

Scope

Review PR #22 after #21 lands. Validate text-layer anchors across zoom, navigation, restart, and representative PDFs. Verify migration/IPC/preload/UI and stale-anchor behavior.

Acceptance criteria

• Selecting text and choosing each color creates exact visible highlight.
• Highlight quote, page, color, anchor, and paper ownership persist.
• Annotation click navigates to correct page and visible range.
• Delete removes list entry and overlay.
• Zoom and restart preserve anchor.
• Invalid/stale anchors degrade without crash or wrong-text highlight.
• Highlights never cross paper boundaries.
• Migration and cascade deletion are proven.

Live verification

Use multi-page real PDF. Highlight beginning/end of lines, multi-line selection, repeated text, then zoom, navigate, restart, jump, and delete. Record any PDF.js selection limitations.

Required PR evidence

PR must use Closes #30, stay focused, and include:

• regression/failing test or reproducible baseline;
• focused tests plus npm test, npm run typecheck, npm run build, git diff --check;
• exact Electron clicks/typing and observed result;
• screenshots for UI changes when tooling permits;
• console output, restart/persistence result, and unresolved limitations;
• real-paper evidence for reader behavior.

Existing PR code and mocked tests are not sufficient. Reviewer must independently exercise live flow before merge.