Edit file: built-in editor + external editor with watch & auto-upload

- Built-in editor (default): large modal with monospace textarea, Cmd+S save,
  dirty guard; reads/writes through the provider streams so it works on local
  and remote files identically (2MB cap with a friendly error)
- External editor: remote files download to appDataDir/edits/<id>, open via
  macOS `open -a`, the temp DIR is watched (survives atomic rename-saves) and
  every save enqueues a normal upload job (progress/errors in the transfer
  list); watchers + temp dirs are cleaned when the session disconnects
- Editor setting: Built-in | Custom with a NATIVE app picker (electrobun
  openFileDialog, /Applications, .app filter) and reset-to-default
- Leftover temp files from a crash prompt for cleanup at boot
- F4, toolbar button (mynaFileText), and File > Edit File (Cmd+E) menu entry
- README: feature notes + Portability notes (open/start/xdg-open seam
  documented in code and README per user request)

Verified headless against the box: sftp read/write roundtrip incl. UTF-8,
size cap, watch->save->auto-upload->remote-updated, session cleanup, orphan
listing and clearing.

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

ui/src/app/app.ts

@@ -5,6 +5,7 @@ import { ConnectionManager } from './connection-manager/connection-manager';
 import { ConnectionsService } from './connections.service';
 import { ChmodDialog } from './dialogs/chmod-dialog';
 import { ConfirmDialog } from './dialogs/confirm-dialog';
+import { EditorDialog } from './dialogs/editor-dialog';
 import { OverwriteDialog } from './dialogs/overwrite-dialog';
 import { PromptDialog } from './dialogs/prompt-dialog';
 import { DialogsService } from './dialogs.service';
@@ -28,6 +29,7 @@ import { TransferBar } from './transfer-bar/transfer-bar';
     ConfirmDialog,
     ChmodDialog,
     OverwriteDialog,
+    EditorDialog,
     TransferBar,
     ConnectionManager,
   ],
@@ -46,9 +48,25 @@ export class App {
     document.title = BRANDING.appName;
     if (this.rpc.available) {
       this.rpc.onMenuAction((action) => this.handleMenuAction(action));
+      void this.promptForEditLeftovers();
     }
   }
 
+  /** Temp files from external edits in a previous run (crash / quit mid-edit). */
+  private async promptForEditLeftovers(): Promise<void> {
+    const res = await this.rpc.request.getEditLeftovers({});
+    if (!res.ok || !res.names.length) return;
+    const shown = res.names.slice(0, 5).join(', ');
+    const more = res.names.length > 5 ? `, +${res.names.length - 5} more` : '';
+    this.dialogs.confirm.set({
+      title: 'Leftover edited files',
+      body: `A previous session left ${res.names.length} temporary edited file(s) behind (${shown}${more}). Clear them?`,
+      confirmLabel: 'Clear',
+      destructive: true,
+      onConfirm: () => void this.rpc.request.clearEditLeftovers({}),
+    });
+  }
+
   /** Native application-menu actions forwarded from the bun process. */
   private handleMenuAction(action: string): void {
     if (action.startsWith('connect:')) {
@@ -72,6 +90,9 @@ export class App {
       case 'new-file':
         this.actions.createFile(this.panels.active());
         break;
+      case 'edit-file':
+        void this.actions.edit(this.panels.active());
+        break;
       case 'refresh-active':
         void this.panels.active().refresh();
         break;