feat(api): Add envelope endpoint and improve API documentation

- Add GET /envelope/{account_id}/{message_id} endpoint to retrieve message envelope (metadata)
- Add get_envelope_by_id method to ENVELOPE_INDEX_MANAGER for querying single envelope
- Move message_id from query parameter to path parameter for clearer API paths:
  - /message-content/{account_id}/{message_id}
  - /download-message/{account_id}/{message_id}
  - /download-attachment/{account_id}/{message_id}
  - /envelope/{account_id}/{message_id}
- Fix API documentation descriptions to be more accurate:
  - search_messages: Now correctly describes search functionality
  - get_thread_messages: Mentions thread_id requirement
  - proxy endpoints: Fixed copy-paste errors from OAuth2 docs
- Update frontend API client to use new path-based URLs

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Cargo.lock

@@ -424,7 +424,7 @@ dependencies = [
 
 [[package]]
 name = "bichon"
-version = "0.1.3"
+version = "0.1.4"
 dependencies = [
  "ahash",
  "async-imap",