feat: add sidebar brand customization

The primary sidebar brand was fixed to the built-in name, subtitle, and logo. Appearance settings now let users preview and save a custom brand name, localized Chinese and English subtitles, and a validated local PNG, JPEG, or WebP logo, with default-logo and full-reset actions.

Customization remains device-local and limited to the top-left brand area; installer, update, Agent, and protocol identities remain GoodBuddy. The desktop package version is now 0.11.6.

Release note: 新增“外观 → 品牌定制”，可修改左上角品牌名称、中英文副标题和本地 Logo，并支持实时预览、恢复默认及本机持久化。

src/main/file-media-type.ts

@@ -1,4 +1,5 @@
 import { extname } from 'node:path'
+export { detectSupportedImage } from '../shared/image-media-type'
 
 const mimeTypes: Readonly<Record<string, string>> = {
   '.c': 'text/x-c',
@@ -40,33 +41,3 @@ export function mimeTypeFromFileName(
 ): string {
   return mimeTypes[extname(name).toLocaleLowerCase()] ?? fallback
 }
-
-export function detectSupportedImage(data: Buffer): {
-  extension: 'jpg' | 'png' | 'webp'
-  mimeType: 'image/jpeg' | 'image/png' | 'image/webp'
-} {
-  if (
-    data.byteLength >= 3 &&
-    data[0] === 0xff &&
-    data[1] === 0xd8 &&
-    data[2] === 0xff
-  ) {
-    return { extension: 'jpg', mimeType: 'image/jpeg' }
-  }
-  if (
-    data.byteLength >= 8 &&
-    data.subarray(0, 8).equals(
-      Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])
-    )
-  ) {
-    return { extension: 'png', mimeType: 'image/png' }
-  }
-  if (
-    data.byteLength >= 12 &&
-    data.subarray(0, 4).toString('ascii') === 'RIFF' &&
-    data.subarray(8, 12).toString('ascii') === 'WEBP'
-  ) {
-    return { extension: 'webp', mimeType: 'image/webp' }
-  }
-  throw new Error('图片格式不受支持')
-}