feat: cross-device session continuity — resume conversation across desktop and mobile

[devartifex]

Problem

When switching between devices (desktop → mobile or vice versa), the user starts with an empty conversation rather than being able to continue from where they left off.

Currently:

• Each browser has its own tabId (UUID stored in localStorage) and its own express-session cookie
• Chat state is stored per userId + tabId, so desktop and mobile have entirely separate histories
• There is no concept of a "current conversation" that spans devices

Proposed Solution

Add a primary session concept per GitHub user that allows any device to load and resume the most recently active conversation.

Phase 3a — Shared Conversation History (read)

1. Primary session metadata file per user: {CHAT_STATE_PATH}/{userId}/primary-session.json

   • Contains: { tabId, sdkSessionId, model, mode, updatedAt }
   • Updated whenever a new message is added to the active conversation

2. On WS connect, if the connecting device has no local chat history for its tabId:

   • Check for a primary session owned by the user
   • If found and recent (< 24h), offer to resume: send a primary_session_available message with the history
   • Client shows a "Continue your conversation" prompt, or auto-loads if the device has never chatted

3. UI change: new ContinueSession banner component or modal

Phase 3b — Active SDK Session Resume

4. SDK session resume across devices:

   • Store the Copilot SDK sessionId in the primary session metadata
   • When reconnecting, call client.resumeSession(sdkSessionId) to reconnect to the same backend session
   • Handle gracefully when the SDK session has expired (fall back to new session)

5. Conflict resolution:

   • Pool key is still : — each device has its own WS pool entry
   • If both devices try to send simultaneously, the second request is queued (already handled by isProcessing)
   • Add a session_taken server message when another device is actively processing, with a short UI indicator

Phase 3c — Real-time Sync (optional enhancement)

6. Broadcast all assistant messages to all connected devices for the same user (fan-out in poolSend)
   • Both desktop and mobile update in real-time when a response arrives on either

Files to Create/Modify

File	Change
src/lib/server/chat-state-store.ts	Add setPrimarySession() / getPrimarySession()
src/lib/server/ws/handler.ts	Check primary session on connect; send primary_session_available
src/lib/server/ws/session-events.ts	Update primary session metadata on turn_end
src/lib/types/index.ts	Add primary_session_available, session_taken message types
src/lib/stores/chat.svelte.ts	Handle primary_session_available message
src/lib/components/	New ContinueSessionBanner.svelte component

Notes

• The tabId in localStorage is per-browser-profile (not per-tab). Desktop and mobile generate independent UUIDs — no conflict.
• This is purely additive — existing per-device behavior is unchanged for users who don't want cross-device sync.
• Phase 3a alone provides significant value and is low-risk.
• Auth persistence (PR fix: auth persistence via encrypted cookie + notification system hardening #120) is a prerequisite — cross-device is only useful if auth survives device switches.

[devartifex]

📝 Repository reset notice: This repo was reset to v1.0.0 with a clean single-commit history. All prior commit references are now orphaned. This issue remains valid and is tracked for future work.