feat: add schema browser sidebar and app layout

Add SchemaTree with database/schema/category hierarchy, Sidebar with
search, Toolbar, TabBar, StatusBar, and full app layout with
resizable panels.

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

src/components/layout/Toolbar.tsx

@@ -0,0 +1,86 @@
+import { useState } from "react";
+import { Button } from "@/components/ui/button";
+import { Separator } from "@/components/ui/separator";
+import { ConnectionSelector } from "@/components/connections/ConnectionSelector";
+import { ConnectionList } from "@/components/connections/ConnectionList";
+import { ConnectionDialog } from "@/components/connections/ConnectionDialog";
+import { useAppStore } from "@/stores/app-store";
+import { Database, Plus } from "lucide-react";
+import type { ConnectionConfig, Tab } from "@/types";
+
+export function Toolbar() {
+  const [listOpen, setListOpen] = useState(false);
+  const [dialogOpen, setDialogOpen] = useState(false);
+  const [editingConn, setEditingConn] = useState<ConnectionConfig | null>(null);
+  const { activeConnectionId, addTab } = useAppStore();
+
+  const handleNewQuery = () => {
+    if (!activeConnectionId) return;
+    const tab: Tab = {
+      id: crypto.randomUUID(),
+      type: "query",
+      title: "New Query",
+      connectionId: activeConnectionId,
+      sql: "",
+    };
+    addTab(tab);
+  };
+
+  return (
+    <>
+      <div className="flex h-10 items-center gap-2 border-b px-3 bg-card">
+        <Button
+          variant="ghost"
+          size="sm"
+          className="h-7 gap-1.5"
+          onClick={() => setListOpen(true)}
+        >
+          <Database className="h-3.5 w-3.5" />
+          Connections
+        </Button>
+
+        <Separator orientation="vertical" className="h-5" />
+
+        <ConnectionSelector />
+
+        <Separator orientation="vertical" className="h-5" />
+
+        <Button
+          variant="ghost"
+          size="sm"
+          className="h-7 gap-1.5"
+          onClick={handleNewQuery}
+          disabled={!activeConnectionId}
+        >
+          <Plus className="h-3.5 w-3.5" />
+          New Query
+        </Button>
+
+        <div className="flex-1" />
+
+        <span className="text-xs font-semibold text-muted-foreground tracking-wide">
+          TUSK
+        </span>
+      </div>
+
+      <ConnectionList
+        open={listOpen}
+        onOpenChange={setListOpen}
+        onEdit={(conn) => {
+          setEditingConn(conn);
+          setDialogOpen(true);
+        }}
+        onNew={() => {
+          setEditingConn(null);
+          setDialogOpen(true);
+        }}
+      />
+
+      <ConnectionDialog
+        open={dialogOpen}
+        onOpenChange={setDialogOpen}
+        connection={editingConn}
+      />
+    </>
+  );
+}