fix(security): harden management panel asset updater

- Abort update when SHA256 digest mismatch is detected instead of
  logging a warning and proceeding (prevents MITM asset replacement)
- Cap asset download size to 10 MB via io.LimitReader (defense-in-depth
  against OOM from oversized responses)
- Add `auto-update-panel` config option (default: false) to make the
  periodic background updater opt-in; the panel is still downloaded
  on first access when missing, but no longer silently auto-updated
  every 3 hours unless explicitly enabled

config.example.yaml

@@ -25,6 +25,10 @@ remote-management:
   # Disable the bundled management control panel asset download and HTTP route when true.
   disable-control-panel: false
 
+  # Enable automatic periodic background updates of the management panel from GitHub (default: false).
+  # When disabled, the panel is only downloaded on first access if missing, and never auto-updated afterward.
+  # auto-update-panel: false
+
   # GitHub repository for the management control panel. Accepts a repository URL or releases API URL.
   panel-github-repository: "https://github.com/router-for-me/Cli-Proxy-API-Management-Center"