feat(ai/stt): add dictation language selection to STT settings

Add a per-workspace `sttLanguage` setting (ISO-639-1 hint; empty =
auto-detect) and a searchable language picker in the Voice / STT settings
card. The hint is forwarded to the transcription endpoint:
- multipart path via the AI SDK `providerOptions.openai.language`
- JSON (OpenRouter) path via a top-level `language` body field
only when non-empty, so auto-detect behaves exactly as before.

Threaded through the whole stack: ai.types, update DTO, AiSettingsService
(resolve/getMasked/update), the workspace.repo SQL allowlist, the client
ai-settings service types, and the provider-settings form. Adds en-US
source keys and ru-RU translations.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -33,6 +33,8 @@ export interface IAiSettings {
   sttModel?: string;
   sttBaseUrl?: string;
   sttApiStyle?: SttApiStyle;
+  // ISO-639-1 dictation language; empty = auto-detect.
+  sttLanguage?: string;
   hasSttApiKey: boolean;
   // RAG indexing coverage (pages indexed for semantic search).
   indexedPages: number;
@@ -60,6 +62,8 @@ export interface IAiSettingsUpdate {
   sttModel?: string;
   sttBaseUrl?: string;
   sttApiStyle?: SttApiStyle;
+  // ISO-639-1 dictation language; empty = auto-detect.
+  sttLanguage?: string;
   // Write-only STT key (same semantics as `apiKey` / `embeddingApiKey`).
   sttApiKey?: string;
 }