Release for v1.8.0

• Added secure Dropbox PKCE authentication with automatic callback handling and a manual fallback.
• Added custom AES-256-GCM encryption with operating-system keychain storage and legacy-data compatibility.
• Added a secure unlock flow for encrypted Dropbox data to prevent accidental cloud overwrites.
• Improved conflict detection, safe conflict backups, synchronization pausing, and local snapshot recovery.
• Added toolbar and command-palette actions for syncing and opening Cloud Sync settings.
• Prevented conflicts with Tabby's built-in configuration sync and improved retry behavior after repeated failures.
• Expanded provider disconnect cleanup while keeping cloud files untouched.
• Improved the settings interface, accessibility, synchronization status, publishing workflow, and automated test coverage.