refactor(proxy): remove is_proxy_target in favor of failover_queue

- Remove `is_proxy_target` field from Provider struct (Rust & TypeScript)
- Remove related DAO methods: get_proxy_target_provider, set_proxy_target
- Remove deprecated Tauri commands: get_proxy_targets, set_proxy_target
- Add `is_available()` method to CircuitBreaker for availability checks
  without consuming HalfOpen probe permits (used in select_providers)
- Keep `allow_request()` for actual request gating with permit tracking
- Update stream_check to use failover_queue instead of is_proxy_target
- Clean up commented-out reset circuit breaker button in ProviderActions
- Remove unused useProxyTargets and useSetProxyTarget hooks

src-tauri/src/deeplink/provider.rs

@@ -132,7 +132,6 @@ pub(crate) fn build_provider_from_request(
         meta,
         icon: request.icon.clone(),
         icon_color: None,
-        is_proxy_target: None,
     };
 
     Ok(provider)

src-tauri/src/deeplink/tests.rs

@@ -145,6 +145,13 @@ fn test_build_gemini_provider_with_model() {
         content: None,
         description: None,
         enabled: None,
+        usage_enabled: None,
+        usage_script: None,
+        usage_api_key: None,
+        usage_base_url: None,
+        usage_access_token: None,
+        usage_user_id: None,
+        usage_auto_interval: None,
     };
 
     let provider = build_provider_from_request(&AppType::Gemini, &request).unwrap();
@@ -191,6 +198,13 @@ fn test_build_gemini_provider_without_model() {
         content: None,
         description: None,
         enabled: None,
+        usage_enabled: None,
+        usage_script: None,
+        usage_api_key: None,
+        usage_base_url: None,
+        usage_access_token: None,
+        usage_user_id: None,
+        usage_auto_interval: None,
     };
 
     let provider = build_provider_from_request(&AppType::Gemini, &request).unwrap();
@@ -232,6 +246,13 @@ fn test_parse_and_merge_config_claude() {
         content: None,
         description: None,
         enabled: None,
+        usage_enabled: None,
+        usage_script: None,
+        usage_api_key: None,
+        usage_base_url: None,
+        usage_access_token: None,
+        usage_user_id: None,
+        usage_auto_interval: None,
     };
 
     let merged = parse_and_merge_config(&request).unwrap();
@@ -275,6 +296,13 @@ fn test_parse_and_merge_config_url_override() {
         content: None,
         description: None,
         enabled: None,
+        usage_enabled: None,
+        usage_script: None,
+        usage_api_key: None,
+        usage_base_url: None,
+        usage_access_token: None,
+        usage_user_id: None,
+        usage_auto_interval: None,
     };
 
     let merged = parse_and_merge_config(&request).unwrap();