fix: saasStore require() bug + health check pool formula + DEV error details

- saasStore.ts: replace require('./chat/conversationStore') with await import()
  to fix ReferenceError in Vite ESM environment (P1)
- main.rs: fix health check pool usage formula from max_connections - num_idle
  to pool.size() - num_idle, preventing false "degraded" status (P1)
- error.rs: show detailed error messages in ZCLAW_SAAS_DEV=true mode
- Update bug tracker with BUG-003 through BUG-007

desktop/src/store/saasStore.ts

@@ -478,7 +478,7 @@ export const useSaaSStore = create<SaaSStore>((set, get) => {
         // switch to the first available model to prevent 404 errors
         if (models.length > 0) {
           try {
-            const { useConversationStore } = require('./chat/conversationStore');
+            const { useConversationStore } = await import('./chat/conversationStore');
             const current = useConversationStore.getState().currentModel;
             const modelIds = models.map(m => m.alias || m.id);
             if (current && !modelIds.includes(current)) {