feat(ui): paint VK status bar to the app theme (VKWebAppSetViewSettings)

Parity with the Telegram chrome painting: on a VK Mini App launch and on
every theme change, set VK's status-bar appearance (and, on Android, the
action/navigation bar colours) from the app's live theme tokens, so the VK
chrome matches the UI instead of clashing.

syncVKChrome mirrors syncTelegramChrome; the status-bar appearance is
derived from the --bg token's luminance (appearanceForBg, unit-tested).
Wired into the VK onScheme handler (fires on launch + on theme change) and
setTheme. Docs: UI_DESIGN VK integration.

ui/src/lib/vk.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { insideVK, onVKPath, vkAppId, vkLaunchParams, vkStartParam } from './vk';
+import { insideVK, onVKPath, vkAppId, vkLaunchParams, vkStartParam, appearanceForBg } from './vk';
 
 describe('vk launch detection', () => {
   afterEach(() => vi.unstubAllGlobals());
@@ -49,3 +49,25 @@ describe('vk launch params', () => {
     expect(vkStartParam()).toBe('');
   });
 });
+
+describe('appearanceForBg', () => {
+  it('maps a dark background to the dark appearance (light status-bar icons)', () => {
+    expect(appearanceForBg('#0f1115')).toBe('dark');
+    expect(appearanceForBg('#171a21')).toBe('dark');
+  });
+
+  it('maps a light background to the light appearance (dark status-bar icons)', () => {
+    expect(appearanceForBg('#f3f4f6')).toBe('light');
+    expect(appearanceForBg('#ffffff')).toBe('light');
+  });
+
+  it('accepts shorthand hex and surrounding whitespace (a raw CSS custom-property value)', () => {
+    expect(appearanceForBg('#000')).toBe('dark');
+    expect(appearanceForBg('  #fff  ')).toBe('light');
+  });
+
+  it('defaults to light for an unparseable colour', () => {
+    expect(appearanceForBg('')).toBe('light');
+    expect(appearanceForBg('rgb(0, 0, 0)')).toBe('light');
+  });
+});