feat(ui): add cross-platform window control styles

Persist automatic and explicit macOS, Windows, GNOME, and minimal control presets with localized settings and platform-aware fallback.

Refs #31

src/utils/windowControlStyle.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, it } from 'vitest';
+import { resolveWindowControlStyle } from './windowControlStyle';
+
+describe('resolveWindowControlStyle', () => {
+  it('uses the platform convention for automatic style', () => {
+    expect(resolveWindowControlStyle('auto', 'macos')).toBe('macos');
+    expect(resolveWindowControlStyle('auto', 'windows')).toBe('windows');
+    expect(resolveWindowControlStyle('auto', 'linux')).toBe('gnome');
+  });
+
+  it('falls back to macOS styling while an unsupported platform is unresolved', () => {
+    expect(resolveWindowControlStyle('auto', 'unknown')).toBe('macos');
+    expect(resolveWindowControlStyle('auto', 'android')).toBe('macos');
+  });
+
+  it('uses the desktop user agent while native platform detection is unresolved', () => {
+    expect(resolveWindowControlStyle('auto', 'unknown', 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)')).toBe('windows');
+    expect(resolveWindowControlStyle('auto', 'unknown', 'Mozilla/5.0 (X11; Linux x86_64)')).toBe('gnome');
+    expect(resolveWindowControlStyle('auto', 'unknown', 'Mozilla/5.0 (Linux; Android 14)')).toBe('macos');
+  });
+
+  it('preserves an explicit style across platforms', () => {
+    expect(resolveWindowControlStyle('macos', 'windows')).toBe('macos');
+    expect(resolveWindowControlStyle('windows', 'linux')).toBe('windows');
+    expect(resolveWindowControlStyle('gnome', 'macos')).toBe('gnome');
+    expect(resolveWindowControlStyle('minimal', 'windows')).toBe('minimal');
+  });
+});