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>

src/modules/rest/api/system.rs

@@ -78,7 +78,7 @@ impl SystemApi {
     #[oai(path = "/proxy/:id", method = "delete", operation_id = "remove_proxy")]
     async fn remove_proxy(
         &self,
-        /// The name of the OAuth2 configuration to retrieve
+        /// The ID of the proxy configuration to delete.
         id: Path<u64>,
         context: ClientContext,
     ) -> ApiResult<()> {
@@ -86,11 +86,11 @@ impl SystemApi {
         Ok(Proxy::delete(id.0).await?)
     }
 
-    /// Retrieve a specific proxy configuration by ID
+    /// Retrieve a specific proxy configuration by ID. Requires root permission.
     #[oai(path = "/proxy/:id", method = "get", operation_id = "get_proxy")]
     async fn get_proxy(
         &self,
-        /// The name of the OAuth2 configuration to retrieve
+        /// The ID of the proxy configuration to retrieve.
         id: Path<u64>,
         context: ClientContext,
     ) -> ApiResult<Json<Proxy>> {