Section editor, activity answers, and UX improvements

[amoghbanta]

Summary

• Section editor — full layout editing in the page detail "By Section" tab: reorder, change type (all 20 types), toggle pruning, delete, add new sections, merge adjacent sections, edit background/text colors, and manage parts (view, remove, add with auto-move between sections)
• PUT /sectioning API endpoint — persists section edits as new versioned page-sectioning data; re-render automatically picks up changes
• Text group editor enhancements — add/remove text groups and text entries, editable group type, delete groups
• Activity answer display — show activity answers and reasoning in both page detail (By Section tab) and storyboard preview panel
• Fix: pipeline status resetting after storyboard accept — stale pipeline-status query cache caused phantom SSE reconnection showing all steps as "Waiting"; now properly invalidated on pipeline complete and storyboard accept
• Export ZIP moved to top bar — removed the green "Storyboard accepted" card, placed Export ZIP button in the header alongside Storyboard button
• Keyboard navigation — left/right arrow keys navigate between pages on the page detail view (disabled when focus is in input fields)

Test plan

• pnpm typecheck passes clean
• pnpm test — all 322 tests pass
• Non-editing mode: By Section tab renders identically to before (activity badges, answers, colors)
• Edit mode: section editor shows with all controls (reorder, type dropdown, prune toggle, colors, parts, merge, delete, add)
• Save section edits → re-render → changes reflected in output
• Add new text group → assign to section → save → re-render
• Accept storyboard → navigate back → pipeline status shows correctly (no "Waiting" reset)
• Export ZIP button visible in top bar when storyboard accepted
• Arrow key navigation between pages (left/right), disabled in input fields