feat(ai): redesign AI settings page with per-endpoint test buttons

Rebuild the workspace AI settings page into card-based "Endpoints"
(Chat / Embeddings / Voice) matching the new design, and split the
single connection test into independent per-endpoint Test buttons.

- server: testConnection(workspaceId, capability) probes only the
  requested capability ('chat' | 'embeddings'); add TestAiConnectionDto
  and wire it through the /workspace/ai-settings/test controller
- client: testAiConnection(capability) + capability-typed mutation; two
  independent test mutation instances so Chat/Embeddings results are isolated
- client: full rewrite of ai-provider-settings into Endpoints section —
  drop the provider dropdown (driver is always openai, base URL + key
  always shown), move the "AI chat" and surface the "Semantic search"
  feature toggles into card headers, system message behind an Edit modal,
  pgvector/reindex footer, and a disabled Voice/STT stub
- client: restyle external MCP tools and the MCP server section; collapse
  the AI sections in workspace-settings; remove the standalone
  ai-chat-settings component
- toggles now surface the server error message (e.g. missing pgvector)
- i18n: add new English strings

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -1148,5 +1148,36 @@
   "Resolved comment": "Resolved comment",
   "Ran tool {{name}}": "Ran tool {{name}}",
   "AI-agent": "AI-agent",
-  "Edited by AI agent on behalf of {{name}}": "Edited by AI agent on behalf of {{name}}"
+  "Edited by AI agent on behalf of {{name}}": "Edited by AI agent on behalf of {{name}}",
+  "Endpoints": "Endpoints",
+  "where we fetch models": "where we fetch models",
+  "All endpoints are OpenAI-compatible. Point the Base URL at OpenAI, OpenRouter, a local Ollama, or any self-hosted server.": "All endpoints are OpenAI-compatible. Point the Base URL at OpenAI, OpenRouter, a local Ollama, or any self-hosted server.",
+  "Chat / LLM": "Chat / LLM",
+  "root": "root",
+  "Semantic search": "Semantic search",
+  "Voice / STT": "Voice / STT",
+  "Voice dictation": "Voice dictation",
+  "Voice dictation is not available yet.": "Voice dictation is not available yet.",
+  "Test endpoint": "Test endpoint",
+  "Save endpoints": "Save endpoints",
+  "External tools": "External tools",
+  "Gitmost as MCP client": "Gitmost as MCP client",
+  "Servers the agent calls out to.": "Servers the agent calls out to.",
+  "MCP server": "MCP server",
+  "expose the workspace": "expose the workspace",
+  "Enable MCP server": "Enable MCP server",
+  "Exposes the workspace as an MCP server at /mcp — this provides a capability, it doesn't consume a model.": "Exposes the workspace as an MCP server at /mcp — this provides a capability, it doesn't consume a model.",
+  "Resolves to {{url}}": "Resolves to {{url}}",
+  "Model": "Model",
+  "Done": "Done",
+  "shared prompt · safety framework appended automatically": "shared prompt · safety framework appended automatically",
+  "/v1/chat/completions · root endpoint — Embeddings and Voice inherit its URL and key": "/v1/chat/completions · root endpoint — Embeddings and Voice inherit its URL and key",
+  "/v1/embeddings · embeds pages so semantic search can find them": "/v1/embeddings · embeds pages so semantic search can find them",
+  "/v1/audio/transcriptions · works with local whisper (speaches / faster-whisper-server)": "/v1/audio/transcriptions · works with local whisper (speaches / faster-whisper-server)",
+  "Vector search · requires pgvector": "Vector search · requires pgvector",
+  "Embedding API key": "Embedding API key",
+  "Embeddings": "Embeddings",
+  "Leave empty to use the chat API key": "Leave empty to use the chat API key",
+  "Leave empty to use the chat base URL": "Leave empty to use the chat base URL",
+  "Reindex now": "Reindex now"
 }