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/public/locales/en-US/translation.json

@@ -1221,6 +1221,9 @@
   "How transcription requests are sent to the endpoint": "How transcription requests are sent to the endpoint",
   "OpenAI-compatible (multipart/form-data)": "OpenAI-compatible (multipart/form-data)",
   "OpenRouter (JSON, base64 audio)": "OpenRouter (JSON, base64 audio)",
+  "Dictation language": "Dictation language",
+  "Auto-detect": "Auto-detect",
+  "Spoken language hint sent to the transcription model. Auto-detect lets the model decide.": "Spoken language hint sent to the transcription model. Auto-detect lets the model decide.",
   "Agent role": "Agent role",
   "Universal assistant": "Universal assistant",
   "Add role": "Add role",