feat(ai-chat): external MCP servers admin UI (E3)" -m "Admin 'AI / External tools (MCP)' settings section: list/add/edit/delete

external MCP servers, per-server enable toggle and Test (lists the server's
tools), write-only auth headers (never shown), tool allowlist, and a Tavily
preset (key in the Authorization header, not the URL). Consumes the existing
admin /workspace/ai-mcp-servers endpoints. Fixes a discriminated-union narrowing
type error in the (previously untracked) server form.

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

@@ -689,6 +689,25 @@
   "View the <anchor>API documentation</anchor> for usage details.": "View the <anchor>API documentation</anchor> for usage details.",
   "View the <anchor>MCP documentation</anchor>.": "View the <anchor>MCP documentation</anchor>.",
   "AI / Models": "AI / Models",
+  "AI / External tools (MCP)": "AI / External tools (MCP)",
+  "Add server": "Add server",
+  "Edit server": "Edit server",
+  "Delete server": "Delete server",
+  "Are you sure you want to delete this MCP server?": "Are you sure you want to delete this MCP server?",
+  "No external servers configured": "No external servers configured",
+  "Server name": "Server name",
+  "Transport": "Transport",
+  "URL": "URL",
+  "Authorization header": "Authorization header",
+  "Tool allowlist": "Tool allowlist",
+  "Optional. Leave empty to allow all tools the server exposes.": "Optional. Leave empty to allow all tools the server exposes.",
+  "Use Tavily preset": "Use Tavily preset",
+  "Test": "Test",
+  "Available tools": "Available tools",
+  "No tools available": "No tools available",
+  "Created successfully": "Created successfully",
+  "Deleted successfully": "Deleted successfully",
+  "Clear": "Clear",
   "Provider": "Provider",
   "•••• set": "•••• set",
   "Clear key": "Clear key",