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/rpc-handlers.ts

@@ -1,6 +1,7 @@
-import { BrowserView } from "electrobun/bun";
+import { BrowserView, Utils } from "electrobun/bun";
 import type { AppRPC } from "../shared/rpc";
 import { effectiveConcurrency, type Result } from "../shared/types";
+import { readFileText, writeFileText, type EditManager } from "./edits";
 import type { SessionRegistry } from "./fs/registry";
 import { SftpProvider } from "./fs/sftp";
 import type { ConnectionStore } from "./store/connections";
@@ -17,6 +18,7 @@ export interface AppDeps {
 	engine: TransferEngine;
 	store: ConnectionStore;
 	settings: SettingsStore;
+	edits: EditManager;
 	/** Assigned after the rpc object exists (it needs rpc.send). */
 	events: {
 		sessionClosed: (info: { sessionId: string; reason: string }) => void;
@@ -32,7 +34,7 @@ async function wrap<T extends object>(fn: () => Promise<T> | T): Promise<Result<
 	}
 }
 
-export function createAppRPC({ registry, engine, store, settings, events }: AppDeps) {
+export function createAppRPC({ registry, engine, store, settings, edits, events }: AppDeps) {
 	return BrowserView.defineRPC<AppRPC>({
 		maxRequestTime: MAX_REQUEST_TIME_MS,
 		handlers: {
@@ -45,6 +47,7 @@ export function createAppRPC({ registry, engine, store, settings, events }: AppD
 							if (!sessionId) return;
 							registry.drop(sessionId);
 							engine.failSession(sessionId, reason);
+							edits.stopForSession(sessionId);
 							events.sessionClosed({ sessionId, reason });
 						});
 						sessionId = registry.add(provider);
@@ -55,6 +58,7 @@ export function createAppRPC({ registry, engine, store, settings, events }: AppD
 
 				disconnect: ({ sessionId }) =>
 					wrap(async () => {
+						edits.stopForSession(sessionId);
 						await registry.remove(sessionId);
 						console.log(`[rpc] disconnected ${sessionId}`);
 						return {};
@@ -160,6 +164,43 @@ export function createAppRPC({ registry, engine, store, settings, events }: AppD
 						engine.setConcurrency(effectiveConcurrency(next));
 						return {};
 					}),
+
+				// ---- file editing ----
+				readFile: ({ sessionId, path }) =>
+					wrap(async () => ({
+						content: await readFileText(registry.get(sessionId), path),
+					})),
+
+				writeFile: ({ sessionId, path, content }) =>
+					wrap(async () => {
+						await writeFileText(registry.get(sessionId), path, content);
+						return {};
+					}),
+
+				editExternal: ({ sessionId, path }) =>
+					wrap(() => edits.editExternal(sessionId, path)),
+
+				pickEditorApp: () =>
+					wrap(async () => {
+						const picked = await Utils.openFileDialog({
+							startingFolder: "/Applications",
+							allowedFileTypes: "app",
+							canChooseFiles: true,
+							canChooseDirectory: false,
+							allowsMultipleSelection: false,
+						});
+						const path = picked.find((p) => p.trim().length > 0) ?? null;
+						return { path };
+					}),
+
+				getEditLeftovers: () =>
+					wrap(async () => ({ names: await edits.listLeftovers() })),
+
+				clearEditLeftovers: () =>
+					wrap(async () => {
+						await edits.clearLeftovers();
+						return {};
+					}),
 			},
 		},
 	});