fix: CI lint errors after #397 commits

- flutter analyze: Riverpod 3 dropped StateProvider; replaced
  _dismissedVersionsProvider with a small Notifier<Set<String>>
  + NotifierProvider, mutating via an `add(version)` method.
  Public API (shouldShowUpdateBannerProvider, dismissUpdateProvider)
  unchanged.
- golangci errcheck: defer f.Close() now wrapped in func() { _ = f.Close() }();
  os.Setenv calls in test helper switched to t.Setenv (cleaner — auto-restores
  on test cleanup, no manual Unsetenv needed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/client_assets.go

@@ -89,7 +89,7 @@ func (h *handlers) handleClientAPK(w http.ResponseWriter, r *http.Request) {
 		writeErrWithLog(w, h.logger, "client_apk: open", err)
 		return
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	stat, err := f.Stat()
 	if err != nil {