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/tab-bar/tab-bar.ts

@@ -2,6 +2,7 @@ import { Component, ElementRef, effect, inject, signal, viewChild } from '@angul
 import { NgIcon } from '@ng-icons/core';
 import { PanelsService } from '../panels.service';
 import { IS_MAC_CHROME } from '../platform';
+import { RpcService } from '../rpc.service';
 import { SettingsService } from '../settings.service';
 
 const THEMES = ['system', 'light', 'dark'] as const;
@@ -188,6 +189,55 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
             </div>
           }
 
+          <div class="mt-2.5 flex items-center justify-between gap-2">
+            <span class="text-xs text-muted-foreground">Editor</span>
+            <div class="flex rounded-md border border-border p-0.5">
+              <button
+                (click)="settings.setEditor('builtin')"
+                class="rounded px-2 py-0.5 text-[11px]"
+                [class.bg-accent]="settings.editor() === 'builtin'"
+                [class.text-foreground]="settings.editor() === 'builtin'"
+                [class.text-muted-foreground]="settings.editor() !== 'builtin'"
+              >
+                Built-in
+              </button>
+              <button
+                (click)="settings.setEditor('custom')"
+                class="rounded px-2 py-0.5 text-[11px]"
+                [class.bg-accent]="settings.editor() === 'custom'"
+                [class.text-foreground]="settings.editor() === 'custom'"
+                [class.text-muted-foreground]="settings.editor() !== 'custom'"
+              >
+                Custom
+              </button>
+            </div>
+          </div>
+
+          @if (settings.editor() === 'custom') {
+            <div class="mt-2.5 flex items-center justify-between gap-2 pl-3">
+              <span class="min-w-0 truncate text-xs text-muted-foreground" [title]="settings.editorApp()">
+                {{ editorAppName() }}
+              </span>
+              <div class="flex shrink-0 items-center gap-1">
+                <button
+                  (click)="pickEditorApp()"
+                  class="rounded-md border border-border px-2 py-0.5 text-[11px] hover:bg-accent"
+                >
+                  Choose…
+                </button>
+                @if (settings.editorApp()) {
+                  <button
+                    (click)="settings.setEditorApp('')"
+                    title="Reset to system default"
+                    class="grid size-5 place-items-center rounded text-muted-foreground hover:bg-accent hover:text-foreground"
+                  >
+                    <ng-icon name="mynaX" size="0.7rem" />
+                  </button>
+                }
+              </div>
+            </div>
+          }
+
           <div class="mt-2.5 flex items-center justify-between gap-2">
             <span class="text-xs text-muted-foreground">Finished transfers</span>
             <div class="flex rounded-md border border-border p-0.5">
@@ -219,6 +269,7 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
 export class TabBar {
   readonly panels = inject(PanelsService);
   readonly settings = inject(SettingsService);
+  private readonly rpc = inject(RpcService);
   readonly settingsOpen = signal(false);
   readonly isMacChrome = IS_MAC_CHROME;
   readonly themes = THEMES;
@@ -248,6 +299,19 @@ export class TabBar {
     this.panels.closeWorkspace(id);
   }
 
+  /** "Visual Studio Code.app" -> "Visual Studio Code"; empty -> system default. */
+  editorAppName(): string {
+    const path = this.settings.editorApp();
+    if (!path) return 'System default';
+    const base = path.split('/').filter(Boolean).pop() ?? path;
+    return base.replace(/\.app$/i, '');
+  }
+
+  async pickEditorApp(): Promise<void> {
+    const res = await this.rpc.request.pickEditorApp({});
+    if (res.ok && res.path) this.settings.setEditorApp(res.path);
+  }
+
   /** Insertion line at the drop position (inset shadow: no layout shift). */
   dropIndicatorShadow(index: number): string | null {
     const target = this.dropTarget();