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/lib.rs

@@ -581,7 +581,6 @@ pub fn run() {
             commands::update_provider,
             commands::delete_provider,
             commands::switch_provider,
-            commands::set_proxy_target_provider,
             commands::import_default_config,
             commands::get_claude_config_status,
             commands::get_config_status,
@@ -684,8 +683,6 @@ pub fn run() {
             commands::is_live_takeover_active,
             commands::switch_proxy_provider,
             // Proxy failover commands
-            commands::get_proxy_targets,
-            commands::set_proxy_target,
             commands::get_provider_health,
             commands::reset_circuit_breaker,
             commands::get_circuit_breaker_config,