feat: add per-connection read-only mode

Connections default to read-only. SQL editor wraps queries in a
read-only transaction so PostgreSQL rejects mutations. Data mutation
commands (update_row, insert_row, delete_rows) are blocked at the
Rust layer. Toolbar toggle with confirmation dialog lets users
switch to read-write. Badges shown in workspace, table viewer, and
status bar.

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

src/components/layout/Toolbar.tsx

@@ -4,6 +4,7 @@ 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 { ReadOnlyToggle } from "@/components/layout/ReadOnlyToggle";
 import { useAppStore } from "@/stores/app-store";
 import { Database, Plus } from "lucide-react";
 import type { ConnectionConfig, Tab } from "@/types";
@@ -45,6 +46,10 @@ export function Toolbar() {
 
         <Separator orientation="vertical" className="h-5" />
 
+        <ReadOnlyToggle />
+
+        <Separator orientation="vertical" className="h-5" />
+
         <Button
           variant="ghost"
           size="sm"