perf(relay): full-chain optimization — key pool, model sync, SSE stream

Phase 1 (Key Pool correctness):
- RPM: fixed-minute window → sliding 60s aggregation (prevents 2x burst)
- Remove fallback-to-provider-key bypass when all keys rate-limited
- SSE semaphore: 16→64 permits, cleanup delay 60s→5s
- Default 429 cooldown: 5min→60s (better for Coding Plan quotas)
- Expire old key_usage_window rows on record

Phase 2 (Frontend model sync):
- currentModel empty-string fallback to glm-4-flash-250414 in relay client
- Merge duplicate listModels() calls in connectionStore SaaS path
- Show ModelSelector in Tauri mode when models available
- Clear currentModel on SaaS logout

Phase 3 (Relay performance):
- Key Pool: DashMap in-memory cache (TTL 5s) for select_best_key
- Cache invalidation on 429 marking

Phase 4 (SSE stream):
- AbortController integration for user-cancelled streams
- SSE parsing: split by event boundaries (\n\n) instead of per-line
- streamStore cancelStream adapts to 0-arg and 1-arg cancel fns

desktop/src/store/connectionStore.ts

@@ -441,9 +441,10 @@ export const useConnectionStore = create<ConnectionStore>((set, get) => {
           // Configure the singleton client (cookie auth — no token needed)
           saasClient.setBaseUrl(session.saasUrl);
 
-          // Health check via GET /api/v1/relay/models
+          // Health check + model list: merged single listModels() call
+          let relayModels: Array<{ id: string; alias?: string }> | null = null;
           try {
-            await saasClient.listModels();
+            relayModels = await saasClient.listModels();
           } catch (err: unknown) {
             // Handle expired session — clear auth and trigger re-login
             const status = (err as { status?: number })?.status;
@@ -473,15 +474,8 @@ export const useConnectionStore = create<ConnectionStore>((set, get) => {
             // baseUrl = saasUrl + /api/v1/relay → kernel appends /chat/completions
             // apiKey = SaaS JWT token → sent as Authorization: Bearer <jwt>
 
-            // Fetch available models from SaaS relay (shared by both branches)
-            let relayModels: Array<{ id: string }>;
-            try {
-              relayModels = await saasClient.listModels();
-            } catch {
-              throw new Error('无法获取可用模型列表，请确认管理后台已配置 Provider 和模型');
-            }
-
-            if (relayModels.length === 0) {
+            // Models already fetched during health check above
+            if (!relayModels || relayModels.length === 0) {
               throw new Error('SaaS 平台没有可用模型，请先在管理后台配置 Provider 和模型');
             }