fix: add folder deletion UI and proper attachment cleanup

[kaniamutan14]

Currently, users can create custom folders, but there was no mechanism to remove them. This introduces a folder deletion workflow that ensures data integrity by cleaning up not only the database records but also the associated physical assets in Cloudflare R2 storage.

Proposed Changes

• Frontend (UI/UX):
  • Added a trash icon on hover for custom folders in the Sidebar.
  • Implemented a safety confirmation dialog to prevent accidental data loss.
  • Added redirection logic: if a user deletes the folder they are currently viewing, the app automatically navigates them back to the Inbox.
• Backend (API & DO):
  • MailboxDO: Updated the deleteFolder method to proactively fetch all attachment paths associated with a folder's
    emails before the records are purged via D1 cascade.
  • API Route: Updated the DELETE /folders/:id endpoint to coordinate the "physical" cleanup of R2 blobs,
    preventing storage leaks (orphaned files).
• Maintenance:
  • Pruned several unused dev dependencies (tsx, yaml, get-tsconfig) to streamline the build environment.