feat: multi-connection parallel support

- connection store: connectedIds array, per-connection health/serverInfo maps
- connect() no longer disconnects existing connections
- New switchTo(id), disconnectById(id), isConnectedById(id) methods
- Per-connection health check timers (Map-based)
- ConnectionCard: switch to connected, connect if not, disconnect if active
- MainArea: per-connection tab state (save/restore on switch)
- Connection switcher dropdown in tab bar showing all open connections
- Clean up tab state when connection disconnects

src/components/ConnectionCard.vue

@@ -9,14 +9,19 @@ const emit = defineEmits<{ edit: [conn: ConnectionConfig]; delete: [id: string]
 const connStore = useConnectionStore()
 
 const isActive = computed(() => connStore.activeId === props.connection.id)
+const isConn = computed(() => connStore.isConnectedById(props.connection.id))
 const statusColor = computed(() => {
-  if (isActive.value) return 'var(--success)'
+  if (isConn.value) return 'var(--success)'
   return 'var(--text-muted)'
 })
 
 function handleClick() {
-  if (isActive.value) {
-    connStore.disconnect()
+  if (isConn.value) {
+    if (isActive.value) {
+      connStore.disconnect()
+    } else {
+      connStore.switchTo(props.connection.id)
+    }
   } else {
     connStore.connect(props.connection)
   }