Implement export_and_upload flow with progress streaming

- Add export.rs module for message export to JSON zip format
 - Streams messages from selected chats via QueryContext
 - Groups by chat with metadata (name, identifier, service)
 - Creates zip package in temp directory with manifest
 - Progress callback support for UI updates

- Add upload.rs module for server communication
 - get_presigned_url() fetches upload URL from server
 - upload_file() uploads zip to R2 with progress tracking
 - create_job() notifies server to start processing
 - Configurable server URL (localhost/chattomap.com)

- Wire up full export_and_upload Tauri command
 - Export stage (0-50%)
 - Pre-sign URL stage (50-55%)
 - Upload stage (55-90%)
 - Job creation stage (90-95%)
 - Browser open and completion (95-100%)

- Update frontend to match new API types
 - ChatInfo.id is now number (was string)
 - ExportResult includes results_url field
 - Progress events properly displayed

Dependencies: zip, chrono, tempfile, reqwest, uuid, open

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ndbroadbent