fix(frontend): clear TypeScript strict-mode errors

- vite-env.d.ts now declares ImportMetaEnv with the three VITE_*
  variables the project uses, restoring proper typing for
  import.meta.env in apiClient.ts.
- QualityFlag.tsx widens its 'flags' prop to accept the domain
  QualityFlags type, the loose Record form used in mocks, or null,
  ending the structural-mismatch errors at five callsites
  (DocumentsPage, DocumentViewerPage, QualityControlPage,
  ChunkPreview, SearchResultCard).
- DashboardPage trend callbacks are typed against DashboardStats so
  the implicit-any complaints disappear without weakening intent.

npx tsc --noEmit -> clean. vite build -> ok.

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

frontend/src/vite-env.d.ts

@@ -0,0 +1,11 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_API_BASE_URL?: string;
+  readonly VITE_USE_MOCK?: string;
+  readonly VITE_APP_NAME?: string;
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv;
+}