feat: General mode (Summarize/Rewrite/Translate/Extract) + persisted mode & AI gating

[HooplaHoorah]

What

• Added General mode UI next to Math (textarea + actions).
• Actions: Summarize, Rewrite (tone selector), Translate (language selector), Extract (type selector).
• Persist mode with chrome.storage.sync; default to General.
• Gate all General actions behind AI readiness (mirrors Math gating).
• Pre-disable General buttons via data-ai-only until probe completes.
• Append-only JS to avoid regressions; Math behavior unchanged.

Why

• Expands extension beyond math-only to common web tasks (summary, rewrite, translate, extract).
• Keeps UX consistent: same side panel, same AI readiness signals, mode persistence across tabs.

How to test

1. Load extension (chrome://extensions → Reload).
2. Open the side panel → toggle General | Math.
3. On any page, select text → Send selection → Web-to-Quest (or paste into General input).
4. In General:
   • Click Summarize → result appears in “Result”.
   • Rewrite with different tones (Friendly/Concise/Formal/Simplify) → result updates.
   • Translate (e.g., Spanish) → result updates.
   • Extract (Highlights/Key points/Dates/People/Tasks) → result updates.
5. Toggle Enable AI features off → all General buttons disable; on → re-enable.
6. Switch to Math and verify: Generate steps / Explain this step / Check answer still work.
7. Close & reopen panel → previously selected mode persists.

Notes

• General calls W2Q_AI.summarize/rewrite/translate/extract, or W2Q_AI.general(...), or W2Q_AI.prompt(...) as fallback.
• No changes to manifest beyond existing "storage" permission (already present).
• Non-breaking change; UI wrapped in .for-general and .for-math.

Checklist

• No final answers leak into Steps (Math)
• General buttons disabled until AI ready (and when AI disabled)
• Mode persists between tabs/panel sessions
• Screenshots attached (follow-up)
• README/Docs mention General mode (follow-up)