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/file-actions.service.ts

@@ -4,6 +4,7 @@ import { DialogsService } from './dialogs.service';
 import { joinPath } from './format';
 import { type PanelState } from './panels.service';
 import { RpcService } from './rpc.service';
+import { SettingsService } from './settings.service';
 import { ToastService } from './toast.service';
 
 /**
@@ -15,6 +16,7 @@ export class FileActionsService {
   private readonly rpc = inject(RpcService);
   private readonly toast = inject(ToastService);
   private readonly dialogs = inject(DialogsService);
+  private readonly settings = inject(SettingsService);
 
   /** Selection if non-empty, else the cursor entry. Never the ".." parent row. */
   targets(panel: PanelState): FileEntry[] {
@@ -56,6 +58,42 @@ export class FileActionsService {
     });
   }
 
+  /** Open the cursor file in the built-in editor or the configured external app. */
+  async edit(panel: PanelState): Promise<void> {
+    const entry = panel.sorted()[panel.cursor()];
+    if (!entry || entry.type !== 'file' || entry.name === '..') return;
+
+    if (this.settings.editor() === 'custom') {
+      const res = await this.rpc.request.editExternal({
+        sessionId: panel.sessionId(),
+        path: entry.path,
+      });
+      if (!res.ok) {
+        this.toast.error(res.error);
+      } else if (res.watching) {
+        this.toast.info(`Editing ${entry.name} — saves upload automatically`);
+      }
+      return;
+    }
+
+    const res = await this.rpc.request.readFile({
+      sessionId: panel.sessionId(),
+      path: entry.path,
+    });
+    if (!res.ok) {
+      this.toast.error(res.error);
+      return;
+    }
+    this.dialogs.editor.set({
+      sessionId: panel.sessionId(),
+      path: entry.path,
+      name: entry.name,
+      sessionTitle: panel.title(),
+      content: res.content,
+      panel,
+    });
+  }
+
   rename(panel: PanelState): void {
     const entry = panel.sorted()[panel.cursor()];
     if (!entry || entry.name === '..') return;