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/format.ts

@@ -23,6 +23,11 @@ export function formatDate(epochMs: number): string {
   return DATE_FORMAT.format(new Date(epochMs));
 }
 
+/** POSIX-style join for UI-composed paths (both panels use "/" on macOS + remote). */
+export function joinPath(dir: string, name: string): string {
+  return dir.endsWith('/') ? dir + name : `${dir}/${name}`;
+}
+
 /** 0o755 -> "rwxr-xr-x" */
 export function formatMode(mode: number): string {
   const flags = 'rwxrwxrwx';