P2: file operations with dialogs (mkdir, new file, rename, delete, chmod)

- prompt/confirm/chmod dialogs (signals-driven via DialogsService)
- FileActionsService: acts on selection or cursor row of the active panel
- function bar + F2/F7/F8/Delete keys; delete confirms and recurses
- verified provider ops + walkTree + removeRecursive on a scratch tree

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/panels.service.ts

@@ -149,6 +149,8 @@ export class PanelsService {
   readonly left: PanelState;
   readonly right: PanelState;
   readonly activeSide = signal<Side>('left');
+  /** Bumped to tell the active panel to grab keyboard focus again (e.g. after a dialog closes). */
+  readonly focusTick = signal(0);
 
   constructor(rpc: RpcService, toast: ToastService) {
     const deps = { rpc, toast };
@@ -175,4 +177,8 @@ export class PanelsService {
   other(side: Side): PanelState {
     return this.get(this.otherSide(side));
   }
+
+  refocus(): void {
+    this.focusTick.update((n) => n + 1);
+  }
 }