feat(claude-desktop): add 3P provider switching with proxy gateway

Adds a new ClaudeDesktop AppType that writes Claude Desktop's third-party
inference profile under configLibrary/, sharing _meta.json with other
launchers (Ollama-compatible) so cc-switch can coexist with them.

Two switch modes:
- direct: provider already exposes claude-* / anthropic/claude-* model
  ids on Anthropic Messages, Claude Desktop connects to it directly.
- proxy: cc-switch's local proxy acts as the inference gateway,
  presenting only claude-* route names to Claude Desktop and mapping
  them to real upstream models. Required after Anthropic restricted
  Claude Desktop to claude-family ids.

Backend:
- New module claude_desktop_config with snapshot/rollback, official seed
  bypass, /claude-desktop/v1/{models,messages} routes, and a single
  source of truth for default proxy routes.
- Gateway token persisted in SQLite, validated on every proxied request.
- get_claude_desktop_status surfaces drift signals (stale models,
  missing routes, proxy stopped, base URL mismatch, missing token).

Frontend:
- Slim ClaudeDesktopProviderForm independent from ProviderForm,
  controlled by a top-level appId guard.
- ProviderList banner consumes the status query (5s polling) and
  renders actionable diagnostics.
- ClaudeDesktopRouteToggle in the header to start/stop the local
  gateway without touching takeover state.
- Three-locale i18n synchronised.

src/components/AppSwitcher.tsx

@@ -12,6 +12,7 @@ interface AppSwitcherProps {
 
 const ALL_APPS: AppId[] = [
   "claude",
+  "claude-desktop",
   "codex",
   "gemini",
   "opencode",
@@ -34,6 +35,7 @@ export function AppSwitcher({
   const iconSize = 20;
   const appIconName: Record<AppId, string> = {
     claude: "claude",
+    "claude-desktop": "claude",
     codex: "openai",
     gemini: "gemini",
     opencode: "opencode",
@@ -42,6 +44,7 @@ export function AppSwitcher({
   };
   const appDisplayName: Record<AppId, string> = {
     claude: "Claude",
+    "claude-desktop": "Claude Desktop",
     codex: "Codex",
     gemini: "Gemini",
     opencode: "OpenCode",