Sync section collapse state across devices

[super3]

Summary

• The per-section collapsed/expanded map was only written to localStorage, so a different device or private tab never saw it on refresh.
• Persist collapsedSections on the server (new collapsed_sections JSONB column) alongside todo_markdown, push on toggle, and adopt it during cross-device sync.

Changes

• New migration 005_add-collapsed-sections-to-conversations.cjs adds the JSONB column with '{}'::jsonb default.
• db.js: include collapsed_sections in getConversation and listConversationsByUser; add updateCollapsedSections(sessionId, userId, sections) with the same ownership rule as updateTodoMarkdown.
• server.js: new POST /api/collapsed-sections endpoint; enforces ownership via canAccess, rejects non-object bodies.
• index.html: on heading click, push the updated map to the server alongside the existing saveChats(); mergeServerConversations adopts server-side collapsedSections for both existing and newly-discovered chats.

Test plan

• Toggle a section on device A, refresh device B (or a fresh private tab) and confirm the same sections are collapsed.
• Toggling still works for guest (unauthenticated) sessions.
• Existing chats without the column behave correctly (default {}).
• jest (couldn't run locally due to restricted registry — please verify in CI).

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Generated by Claude Code

[railway-app]

🚅 Deployed to the padtask-pr-27 environment in padtask

Service	Status	Web	Updated (UTC)
padtask	✅ Success (View Logs)	Web	May 21, 2026 at 10:59 pm