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>

scripts/gen-version.ts

@@ -0,0 +1,46 @@
+// Writes src/shared/version.gen.ts (gitignored) with the "Build n" version:
+// n = commit count since the repo root, Sublime Text style. Regenerated by
+// postinstall and by every dev/build/release script — never committed, since
+// committing it would bump the very count it records.
+import path from "node:path";
+
+function git(args: string[]): string | null {
+	try {
+		const proc = Bun.spawnSync(["git", ...args], {
+			cwd: import.meta.dir,
+			stdout: "pipe",
+			stderr: "ignore",
+		});
+		if (!proc.success) return null;
+		return proc.stdout.toString().trim() || null;
+	} catch {
+		return null;
+	}
+}
+
+const number = Number(git(["rev-list", "--count", "HEAD"]) ?? 0);
+const commit = git(["rev-parse", "--short", "HEAD"]) ?? "dev";
+const date = new Date().toISOString().slice(0, 10);
+
+let electrobun = "unknown";
+try {
+	const pkg = await Bun.file(
+		path.join(import.meta.dir, "..", "node_modules", "electrobun", "package.json"),
+	).json();
+	electrobun = pkg.version;
+} catch {
+	// not installed yet (postinstall ordering) — fine
+}
+
+const out = `// GENERATED by scripts/gen-version.ts — do not edit, do not commit.
+export const BUILD = {
+	number: ${number},
+	commit: ${JSON.stringify(commit)},
+	date: ${JSON.stringify(date)},
+	electrobun: ${JSON.stringify(electrobun)},
+} as const;
+`;
+
+const target = path.join(import.meta.dir, "..", "src", "shared", "version.gen.ts");
+await Bun.write(target, out);
+console.log(`[version] Build ${number} (${commit}) -> ${path.relative(process.cwd(), target)}`);