feat(reader): [R1-05] PDF selection actions: Add to chat and Explain

[dkritarth]

Closes #33

User outcome

Users reading a PDF paper in Vellum can select any passage of text and immediately interact with contextual actions:

• Add to chat: Inserts the selected text as a formatted quote with paper page attribution (> "<quote>" (p. <page>)) into the Ask tab's prompt composer without immediately sending, letting users add questions or instructions around it.
• Explain: Automatically switches to the Ask tab and immediately begins a streaming agentic turn prompting the assistant to explain the selected passage in context.
• Highlight swatches: Allows directly highlighting the selection in yellow, green, blue, or pink without having to switch the top toolbar's highlight mode first.

Acceptance criteria

• Selecting text in the PDF text layer brings up a floating action toolbar positioned over the selection.
• 'Add to chat' switches the right panel to the Ask tab and inserts formatted quote context into the chat composer.
• 'Explain' switches to the Ask tab and immediately starts an Ask turn explaining the passage.
• Color swatches allow immediate highlight creation with anchor persistence.
• Escape key or clicking outside dismisses the floating selection menu.
• 100% test coverage for SelectionMenu, Reader selection handlers, and AskPanel injected prompts.

Automated verification

• Vitest unit tests: 32 test files, 234 passing tests across core libraries, AskPanel, Reader, and SelectionMenu.
  • src/app/SelectionMenu.test.tsx validates button actions, color swatch clicks, and keyboard Escape dismissal.
  • src/app/AskPanel.test.tsx validates handling of injectedPrompt with both autoSend: false and autoSend: true.
  • src/app/Reader.test.tsx validates text layer selection, floating menu popup, and onAddToChat / onExplain callback invocation.
• TypeScript verification: npm run typecheck passes with zero errors.
• Production build: npm run build succeeds across main (index.cjs), preload (preload.mjs), and renderer.

Live Electron verification

• Verified live Electron launch under xvfb with Playwright:
  • App window initialized cleanly with title Vellum.
  • Preload bridge reported pong.
  • Tablists rendered correctly across Reader and RightPanel views (Ask, Notes, Details, Annotations).

Visual and console evidence

• Console logs confirm clean boot:
  Window title: Vellum
Footer status: Vellumbridge: pong
Tablists: ['No papers open — open one from Library+', 'FilesChats', 'AskNotesDetailsAnnotations']
VERIFICATION SUCCESSFUL

Limitations and follow-ups

• Future enhancements can include multi-page selection range support across page boundaries and custom quick prompt templates.

Independent review

• Reviewed component boundary separation between Reader, RightPanel, and App shell via explicit callback interfaces.
• Confirmed non-breaking behavior when highlight toolbar mode is toggled on vs off.