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-tauri/src/proxy/server.rs

@@ -285,6 +285,15 @@ impl ProxyServer {
             // Claude API (支持带前缀和不带前缀两种格式)
             .route("/v1/messages", post(handlers::handle_messages))
             .route("/claude/v1/messages", post(handlers::handle_messages))
+            // Claude Desktop 3P 本地 gateway（独立 provider namespace）
+            .route(
+                "/claude-desktop/v1/models",
+                get(handlers::handle_claude_desktop_models),
+            )
+            .route(
+                "/claude-desktop/v1/messages",
+                post(handlers::handle_claude_desktop_messages),
+            )
             // OpenAI Chat Completions API (Codex CLI，支持带前缀和不带前缀)
             .route("/chat/completions", post(handlers::handle_chat_completions))
             .route(