feat: add pre-generation outline review

[nkmohit]

Summary:
Adds an optional "Review Outline" step between outline generation and content generation. Off by default so the existing flow is unchanged for casual users. When enabled, generation pauses after the outline is drafted and shows a review page where users can edit scene titles, descriptions, key points, reorder scenes, change scene types, add new scenes, or remove unwanted ones before confirming and starting content generation.

Related Issues:
Related to #173. Context from discussion in #150 where wyuc noted a mandatory confirmation step added friction this addresses that by making the review step opt-in via a toolbar toggle.

Changes:

• Added "Outline" tab to the generation toolbar popover with a "Review Outline" toggle (off by default)
• Added outline review page that renders OutlinesEditor (previously unused component) after outline streaming completes
• Generation pauses on outline step when toggle is enabled and only proceeds on user confirmation
• Added i18n translations for English and Chinese
• Added persisted setting for the toggle state

Type of Change:

• New feature (non-breaking change that adds functionality)

Steps to reproduce / test:

1. Enable "Review Outline" in the generation toolbar (Image/Video/TTS/ASR/Outline tabs)
2. Enter a topic and click "Enter Classroom"
3. Watch outline generate — generation pauses and shows the review page
4. Edit, reorder, or remove scenes as needed
5. Click "Confirm and generate course" to proceed
6. Verify content generation uses the edited outlines
7. Disable the toggle and confirm generation proceeds automatically as before

What you personally verified:

• Toggle is off by default, existing generation flow unchanged
• Review page renders correctly with all outline items editable
• Scene type dropdown, reorder controls, delete, and add scene all work
• "Back to requirements" returns to the input page
• "Confirm and generate course" proceeds with edited outlines
• i18n works in both English and Chinese
• pnpm exec tsc --noEmit passes

Evidence:

• CI passes (pnpm check && pnpm lint && npx tsc --noEmit)
• Manually tested locally
• Screenshots attached

screenshots:

Checklist:

• My code follows the project's coding style
• I have performed a self-review of my code
• My changes do not introduce new warnings