Project conversations API switched from numeric offsets to opaque
alphanumeric cursors. The old code discarded the cursor and fell back
to offset += limit, so only the first page was ever exported.
- Add `cursor?: string | null` to `ApiConversations` interface
- Update `projectConversationsApi` to accept `string | number` cursor
- Expose the API-returned cursor in `fetchProjectConversations`
- Use cursor-based advancement in `fetchAllConversations` for project
requests, with numeric offset fallback for regular conversations
- Add early-exit guard when API returns neither total nor next cursor
Closes#341
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
