Sublime-style Build-n versioning + About box

Build n = commit count, baked at build time into a gitignored
version.gen.ts (postinstall + all build scripts regenerate it). About
lives in the macOS app menu and as a footer row in the options popover
(the cross-platform path), showing build, commit, date and engines.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/shared/rpc.ts

@@ -137,6 +137,8 @@ export type AppRPC = {
 			moveToTrash: { params: { path: string }; response: Result };
 			/** Native column-picker menu (right-click on the list headers). */
 			showColumnsMenu: { params: { visible: ColumnKey[] }; response: Result };
+			/** Native About box (name, Build n, commit, engine versions). */
+			showAbout: { params: Record<string, never>; response: Result };
 			// editor windows
 			/** Main window only: open (or focus) a native editor window. */
 			openEditor: { params: EditorOpenContext; response: Result };