feat(ai-chat): show RAG indexing coverage in AI settings

Display "Indexed N of M pages" on the AI provider settings page so admins
can see how much of the wiki is covered by vector-RAG semantic search.

- page-embedding.repo: add countIndexedPages() — distinct non-deleted pages
  that have stored embeddings in the workspace
- page.repo: add countByWorkspace() — total non-deleted pages
- ai-settings.service: compute both counts in getMasked() (Promise.all) and
  return them with the masked settings; inject PageEmbeddingRepo + PageRepo
- MaskedAiSettings / IAiSettings: add indexedPages + totalPages
- ai-provider-settings: render a dimmed coverage line under "Embedding model"
- i18n: add the "Indexed {{indexed}} of {{total}} pages" key (en-US, ru-RU)

apps/client/src/features/workspace/services/ai-settings-service.ts

@@ -12,6 +12,9 @@ export interface IAiSettings {
   baseUrl?: string;
   systemPrompt?: string;
   hasApiKey: boolean;
+  // RAG indexing coverage (pages indexed for semantic search).
+  indexedPages: number;
+  totalPages: number;
 }
 
 // Update payload. Key semantics: