[R1-05] Add PDF selection actions: Add to chat and Explain

[dkritarth]

Outcome

Turn selected paper text into immediate research context. User can add selection to Ask composer or start an explanation grounded in paper.

Dependencies

Blocked by #32. Build on verified reader selection/highlight seam; do not mix into PR #22 retroactively.

Scope

• Contextual selection menu with Add to chat and Explain.
• Capture exact quote, paper slug/title, page, and stable location when available.
• Add to chat inserts editable quoted context without sending.
• Explain opens Ask and sends a clear paper-grounded question.
• Preserve existing highlight action and normal copy/selection.
• Define IPC/chat contract test-first before UI wiring.

Acceptance criteria

• Menu appears only for non-empty selection and dismisses predictably.
• Add to chat preserves quote and source/page context, remains editable, does not auto-send.
• Explain sends one turn through selected backend and streams response.
• Prompt instructs agent to interpret selection in paper context and cite relevant section.
• Selection from paper A cannot be sent as paper B context.
• Multi-line/unicode/long selection has defined safe behavior.
• Keyboard and screen-reader path exists.
• Highlight, copy, zoom, search, and citations still work.

Live verification

On real PDF, test short sentence, equation-adjacent text, multi-line passage, repeated text, page switch, Add to chat edit/send, Explain, backend error, and response grounding against source.

Required PR evidence

PR must use Closes #33, 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.