feat(annotations): Sync annotations with cloud storage

[ainergiz]

Summary

Enable cross-device access and backup for bookmark annotations by syncing to cloud storage.

Context

Annotations are currently stored locally at ~/.config/xfeed/annotations.json. This works well for single-device use but doesn't support:

• Accessing annotations from multiple machines
• Backup/restore of annotation data
• Sharing annotation data between devices

Related: #136 (original annotation feature)

Current Behavior

• Annotations stored locally in ~/.config/xfeed/annotations.json
• Data only accessible on the machine where it was created
• No backup mechanism

Expected Behavior

• Option to sync annotations to cloud storage
• Support for major providers (iCloud, Dropbox, Google Drive, or custom)
• Seamless merge of annotations from multiple devices
• Offline-first with sync when online

Potential Implementation

Approach 1: File-based Sync (Symlink)

• Allow users to configure a custom path for annotations.json
• User can point to a cloud-synced folder (Dropbox, iCloud, etc.)
• Files: src/config/annotations.ts, src/config/preferences.ts
• Pros: Simple, user-controlled, works with any sync service
• Cons: Conflict handling left to sync service

Approach 2: Native Cloud Integration

• Direct integration with cloud storage APIs
• Built-in conflict resolution
• Files: New src/services/cloud-sync.ts
• Pros: Better UX, controlled sync behavior
• Cons: More complex, requires API keys/auth

Approach 3: Custom Sync Server

• Self-hosted or managed sync endpoint
• Full control over sync logic
• Pros: Maximum flexibility
• Cons: Requires server infrastructure

Additional Context

Current annotation file structure:

{
  "version": 1,
  "annotations": {
    "tweetId": {
      "text": "annotation text",
      "createdAt": "ISO date",
      "updatedAt": "ISO date"
    }
  }
}