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/shared/rpc.ts

@@ -74,6 +74,28 @@ export type AppRPC = {
 				response: Result<{ settings: AppSettings }>;
 			};
 			saveSettings: { params: { settings: AppSettings }; response: Result };
+			// file editing
+			readFile: {
+				params: { sessionId: string; path: string };
+				response: Result<{ content: string }>;
+			};
+			writeFile: {
+				params: { sessionId: string; path: string; content: string };
+				response: Result;
+			};
+			editExternal: {
+				params: { sessionId: string; path: string };
+				response: Result<{ watching: boolean }>;
+			};
+			pickEditorApp: {
+				params: Record<string, never>;
+				response: Result<{ path: string | null }>;
+			};
+			getEditLeftovers: {
+				params: Record<string, never>;
+				response: Result<{ names: string[] }>;
+			};
+			clearEditLeftovers: { params: Record<string, never>; response: Result };
 		};
 		messages: Record<never, never>;
 	};

src/shared/types.ts

@@ -52,6 +52,10 @@ export interface AppSettings {
 	transferConcurrency: number;
 	/** Remove completed transfers from the list automatically. */
 	autoClearCompleted: boolean;
+	/** Edit files with the built-in editor or an external application. */
+	editor: "builtin" | "custom";
+	/** Full path to the external editor .app; empty = OS default application. */
+	editorApp: string;
 }
 
 export const DEFAULT_SETTINGS: AppSettings = {
@@ -61,6 +65,8 @@ export const DEFAULT_SETTINGS: AppSettings = {
 	transferMode: "single",
 	transferConcurrency: 3,
 	autoClearCompleted: true,
+	editor: "builtin",
+	editorApp: "",
 };
 
 export function effectiveConcurrency(settings: AppSettings): number {