feat(flow-chat): add quote-select — copy or quote selected message text into composer

[buddha68]

Summary

Adds a "select-to-quote" workflow to the chat composer, mirroring the interaction
popularized by qoderwork and dsh's @deepseek-ai/dsh-quote-select:

1. Select text inside a user or assistant message → a floating menu appears with
   Copy text and Add to conversation.
2. Add to conversation writes a provenance marker
   【引用自对话第 N 条消息（你的原话/AI 的回答）】「原文」 into the composer
   (quote first, existing draft below) and renders a removable quote chip above
   the input.
3. Chips stay in sync with the draft: removing the marker text removes the chip;
   sending clears both; clicking ✕ removes the marker from the draft.

Design

• New component src/web-ui/src/flow_chat/components/QuoteSelect/:
  • QuoteSelect.tsx — selection detection + floating menu (portal to
    getAppearanceOverlayHost()), copy & add handlers.
  • quoteSelectPick.ts — pure selection extraction (separate file to satisfy
    react-refresh/only-export-components).
  • quoteSelectStore.ts — zustand store; composer read/write via the existing
    public event-bus contracts.
  • QuoteDock.tsx — removable quote chips rendered through the new
    composerDock extension point.
  • QuoteSelect.scss, QuoteSelect.test.tsx (12 tests).
• Minimal host changes (~15 lines total):
  • ChatInput.tsx: new optional render-prop prop composerDock?: (draft: string) => React.ReactNode,
    rendered above the composer inside bitfun-chat-input__container.
  • ChatPane.tsx: mounts <QuoteSelect /> and passes <QuoteDock draft={...} />
    to ChatInput.
• Reuses existing contracts (no DOM hacks):
  • Message anchors: data-bf-component="user-message-item" (+ data-turn-id) and
    data-bf-component="model-round-item".
  • Composer write: globalEventBus.emit('fill-chat-input', { content, mode }).
  • Composer read: globalEventBus.emit('chat-input:get-state', { getValue }).
• New dev dependencies: @testing-library/react, @testing-library/dom (test-only).

Verification

• vitest run QuoteSelect → 12/12 passed.
• eslint on changed files → 0 errors, 0 warnings.
• Existing ChatInput* test suites → 83/83 passed (no regressions).
• vite dev-server compile check on all touched modules → 200 OK.

Notes / follow-ups

• v1 message ordinal is a DOM-based approximation (visible rows). A follow-up
  could compute global ordinals via absoluteSessionTurnIndexForId and add
  data-turn-id to ModelRoundItem for consistency.
• Tool-output-only quotes and multi-message quotes are out of scope for v1.