Fix hints generation robustness and HokieBird activity UX

[Arnav2525]

Summary

This PR improves hint generation reliability/quality and fixes HokieBird maze UX regressions.

Problem

• Hints could appear stuck on "Generating hints..." on some pages.
• Some generated hints were low-quality (e.g. included "No question found").
• In HokieBird maze activity:
  • students had to use Reset All after one mistake,
  • drag/drop was disabled on some pages due to inconsistent props.draggable values.

What Changed

Backend (backend/src/routers/pages.py)

• Improved hint input extraction in create_page_with_gpt:
  • question now falls back across:
    1. props.question
    2. page.questions[0].question
    3. question-like line from page.content
• Added answer/options fallback from page.questions when props.answers is missing.
• Detects stale cached hints containing "no question found" and regenerates them.
• Improved DEV_MODE mock hints to avoid placeholder garbage text when question is missing.

Frontend (frontend/src/pages/BookPage.tsx)

• Hardened getAllHints():
  • added timeout (Promise.race with 12s timeout),
  • added fallback hints for timeout/error/parse failures/empty results,
  • ensures loading state clears reliably.
• Help me now opens hints window immediately (interaction logging is non-blocking).

Frontend (frontend/src/components/HokieBirdMap.tsx)

• Added per-slot clear action (×) so a single incorrect block can be removed.
• Restored reliable drag/drop:
  • action chips always draggable,
  • drop handlers attached on target input/container for consistent drops.
• Kept click-to-fill behavior and Reset All.

User Impact

• Hint generation is more robust and context-aware on production pages.
• Students no longer get stuck with unusable "No question found" hints.
• Maze activity is less frustrating:
  • can clear one block instead of restarting whole sequence,
  • drag/drop works consistently.

Verification

• Frontend typecheck passes: npx tsc --noEmit
• Manual validation:
  • hints endpoint returns contextual hints on real dataset pages,
  • stale bad hints are regenerated,
  • single-block clear and drag/drop work in HokieBird maze.

Files Changed

• backend/src/routers/pages.py
• frontend/src/pages/BookPage.tsx
• frontend/src/components/HokieBirdMap.tsx