feat: add connection management UI

Add TypeScript types, typed Tauri invoke wrappers, Zustand store,
TanStack Query hooks, and connection components: ConnectionDialog
(create/edit/test), ConnectionList (sheet panel), ConnectionSelector
(toolbar dropdown).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/stores/app-store.ts

@@ -0,0 +1,75 @@
+import { create } from "zustand";
+import type { ConnectionConfig, Tab } from "@/types";
+
+interface AppState {
+  connections: ConnectionConfig[];
+  activeConnectionId: string | null;
+  currentDatabase: string | null;
+  connectedIds: Set<string>;
+  tabs: Tab[];
+  activeTabId: string | null;
+  sidebarWidth: number;
+  pgVersion: string | null;
+
+  setConnections: (connections: ConnectionConfig[]) => void;
+  setActiveConnectionId: (id: string | null) => void;
+  setCurrentDatabase: (db: string | null) => void;
+  addConnectedId: (id: string) => void;
+  removeConnectedId: (id: string) => void;
+  setPgVersion: (version: string | null) => void;
+
+  addTab: (tab: Tab) => void;
+  closeTab: (id: string) => void;
+  setActiveTabId: (id: string | null) => void;
+  updateTab: (id: string, updates: Partial<Tab>) => void;
+  setSidebarWidth: (width: number) => void;
+}
+
+export const useAppStore = create<AppState>((set) => ({
+  connections: [],
+  activeConnectionId: null,
+  currentDatabase: null,
+  connectedIds: new Set(),
+  tabs: [],
+  activeTabId: null,
+  sidebarWidth: 260,
+  pgVersion: null,
+
+  setConnections: (connections) => set({ connections }),
+  setActiveConnectionId: (id) => set({ activeConnectionId: id }),
+  setCurrentDatabase: (db) => set({ currentDatabase: db }),
+  addConnectedId: (id) =>
+    set((state) => ({
+      connectedIds: new Set([...state.connectedIds, id]),
+    })),
+  removeConnectedId: (id) =>
+    set((state) => {
+      const next = new Set(state.connectedIds);
+      next.delete(id);
+      return { connectedIds: next };
+    }),
+  setPgVersion: (version) => set({ pgVersion: version }),
+
+  addTab: (tab) =>
+    set((state) => ({
+      tabs: [...state.tabs, tab],
+      activeTabId: tab.id,
+    })),
+  closeTab: (id) =>
+    set((state) => {
+      const tabs = state.tabs.filter((t) => t.id !== id);
+      const activeTabId =
+        state.activeTabId === id
+          ? tabs.length > 0
+            ? tabs[tabs.length - 1].id
+            : null
+          : state.activeTabId;
+      return { tabs, activeTabId };
+    }),
+  setActiveTabId: (id) => set({ activeTabId: id }),
+  updateTab: (id, updates) =>
+    set((state) => ({
+      tabs: state.tabs.map((t) => (t.id === id ? { ...t, ...updates } : t)),
+    })),
+  setSidebarWidth: (width) => set({ sidebarWidth: width }),
+}));