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>

src/bun/index.ts

@@ -1,6 +1,7 @@
 import { ApplicationMenu, BrowserWindow, Updater } from "electrobun/bun";
 import { BRANDING } from "../shared/branding";
 import { effectiveConcurrency } from "../shared/types";
+import { EditManager } from "./edits";
 import { SessionRegistry } from "./fs/registry";
 import { applyApplicationMenu } from "./menu";
 import { createAppRPC } from "./rpc-handlers";
@@ -32,11 +33,12 @@ const registry = new SessionRegistry();
 const engine = new TransferEngine(registry);
 const store = new ConnectionStore();
 const settings = new SettingsStore();
+const edits = new EditManager(registry, engine, settings);
 const events = {
 	sessionClosed: (_info: { sessionId: string; reason: string }) => {},
 	connectionsChanged: () => {},
 };
-const rpc = createAppRPC({ registry, engine, store, settings, events });
+const rpc = createAppRPC({ registry, engine, store, settings, edits, events });
 const url = await getMainViewUrl();
 
 const isMac = process.platform === "darwin";