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/stores/app-store.ts

@@ -6,6 +6,7 @@ interface AppState {
   activeConnectionId: string | null;
   currentDatabase: string | null;
   connectedIds: Set<string>;
+  readOnlyMap: Record<string, boolean>;
   tabs: Tab[];
   activeTabId: string | null;
   sidebarWidth: number;
@@ -16,6 +17,7 @@ interface AppState {
   setCurrentDatabase: (db: string | null) => void;
   addConnectedId: (id: string) => void;
   removeConnectedId: (id: string) => void;
+  setReadOnly: (connectionId: string, readOnly: boolean) => void;
   setPgVersion: (version: string | null) => void;
 
   addTab: (tab: Tab) => void;
@@ -30,6 +32,7 @@ export const useAppStore = create<AppState>((set) => ({
   activeConnectionId: null,
   currentDatabase: null,
   connectedIds: new Set(),
+  readOnlyMap: {},
   tabs: [],
   activeTabId: null,
   sidebarWidth: 260,
@@ -41,13 +44,19 @@ export const useAppStore = create<AppState>((set) => ({
   addConnectedId: (id) =>
     set((state) => ({
       connectedIds: new Set([...state.connectedIds, id]),
+      readOnlyMap: { ...state.readOnlyMap, [id]: true },
     })),
   removeConnectedId: (id) =>
     set((state) => {
       const next = new Set(state.connectedIds);
       next.delete(id);
-      return { connectedIds: next };
+      const { [id]: _, ...restRo } = state.readOnlyMap;
+      return { connectedIds: next, readOnlyMap: restRo };
     }),
+  setReadOnly: (connectionId, readOnly) =>
+    set((state) => ({
+      readOnlyMap: { ...state.readOnlyMap, [connectionId]: readOnly },
+    })),
   setPgVersion: (version) => set({ pgVersion: version }),
 
   addTab: (tab) =>