Storyboard review page with inline re-render and UX improvements

[amoghbanta]

Summary

• Storyboard review page: Two-panel layout with page sidebar and rendered HTML preview, inline re-render button with success/error banners
• Accept Storyboard: Dialog with pending count badge showing how many pages still need rendering
• Page edit UX: "Back to Storyboard" button, "Save & Re-render" combined action, success banner after re-render
• Disabled button tooltips: All disabled buttons now explain why (no API key, no pipeline data, unsaved changes)
• Settings sheet: Rebuild warning showing how many pages will be affected, pruning controls for text/section types

Test plan

• Storyboard: select a rendered page → see "Re-render" button in preview header
• Click Re-render → spinner → success banner appears
• Accept button shows "N pending" badge when not all pages are rendered
• Page edit: see "← Storyboard" button at top left, navigates back
• Edit mode: "Save & Re-render" button available when changes exist
• Click Save & Re-render → saves then re-renders, success banner appears
• Hover disabled buttons → tooltip explains why they're disabled
• Settings sheet: warning text before Save & Rebuild says "all N pages"
• pnpm typecheck — clean
• pnpm vitest run — 289 tests pass