Fix backup API 404s, hardware status 500, and HTMX loading race

- Add all backup API routes to api_v3.py: preview, list, export,
  validate, restore (with plugin reinstall), download, delete
- Fix PermissionError on /hardware/status: return graceful 200 instead
  of 500 when the status file is owned by a different user; also fix
  root cause by writing the file world-readable (0o644) in display_manager
- Fix HTMX race: dispatch htmx:ready window event from HTMX onload
  callback; loadTabContent now waits for that event instead of
  immediately falling back to direct fetch (eliminating the
  "HTMX not available" console warning on initial load)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/display_manager.py

@@ -190,7 +190,7 @@ class DisplayManager:
                         json.dump(_hw_status, _f)
                         _f.flush()
                         os.fsync(_f.fileno())
-                    os.chmod(_tmp_path, 0o600)
+                    os.chmod(_tmp_path, 0o644)
                     os.replace(_tmp_path, _status_path)
                 except Exception:
                     try: