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 {

internal/api/client_assets_test.go

@@ -17,13 +17,13 @@ func withClientAPKDir(t *testing.T) string {
 	t.Helper()
 	dir := t.TempDir()
 	prev, hadPrev := os.LookupEnv("MINSTREL_CLIENT_APK_DIR")
-	os.Setenv("MINSTREL_CLIENT_APK_DIR", dir)
+	t.Setenv("MINSTREL_CLIENT_APK_DIR", dir)
 	t.Cleanup(func() {
 		if hadPrev {
-			os.Setenv("MINSTREL_CLIENT_APK_DIR", prev)
-		} else {
-			os.Unsetenv("MINSTREL_CLIENT_APK_DIR")
+			t.Setenv("MINSTREL_CLIENT_APK_DIR", prev)
 		}
+		// t.Setenv auto-restores the prior empty/unset state at end of
+		// test, so the !hadPrev branch needs no explicit Unsetenv.
 	})
 	return dir
 }