修复部分自定义表情导出后图裂的问题

校验应用缓存、微信 Emoticon 缓存及 CDN 响应的真实图片格式，无效缓存自动回退下载。

增量导出会识别并替换旧归档中的无效表情资源；刷新失败时移除破图引用并保留明确错误。

补充 StickerService 单元测试和 HTML 媒体增量导出回归测试。

tests/unit/sticker-service.test.ts

@@ -0,0 +1,93 @@
+import crypto from 'crypto'
+import fs from 'fs-extra'
+import http from 'http'
+import os from 'os'
+import path from 'path'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { StickerService } from '../../src/main/sticker-service'
+import type { Wcdb4Client } from '../../src/main/wcdb4-client'
+
+const validGif = Buffer.from(
+  '47494638396101000100800000000000ffffff21f90401000000002c00000000010001000002024401003b',
+  'hex'
+)
+
+describe('StickerService', () => {
+  let homeDir = ''
+  const servers: http.Server[] = []
+
+  beforeEach(() => {
+    homeDir = fs.mkdtempSync(path.join(os.tmpdir(), 'sticker-service-'))
+    vi.stubEnv('HOME', homeDir)
+  })
+
+  afterEach(async () => {
+    vi.unstubAllEnvs()
+    await Promise.all(
+      servers
+        .splice(0)
+        .map(
+          (server) =>
+            new Promise<void>((resolve, reject) =>
+              server.close((error) => (error ? reject(error) : resolve()))
+            )
+        )
+    )
+    fs.removeSync(homeDir)
+  })
+
+  const serve = async (body: Buffer): Promise<{ url: string; requests: () => number }> => {
+    let requestCount = 0
+    const server = http.createServer((_request, response) => {
+      requestCount += 1
+      response.writeHead(200)
+      response.end(body)
+    })
+    servers.push(server)
+    await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve))
+    const address = server.address()
+    if (!address || typeof address === 'string') throw new Error('HTTP fixture did not bind a port')
+    return {
+      url: `http://127.0.0.1:${address.port}/sticker.gif`,
+      requests: () => requestCount
+    }
+  }
+
+  it('ignores invalid app and WeChat caches and falls back to the CDN', async () => {
+    const md5 = crypto.createHash('md5').update(validGif).digest('hex')
+    const invalidCache = Buffer.from('encrypted-wechat-emoticon-cache')
+    const appCache = path.join(homeDir, 'Documents', 'TraceMemo', 'Emojis', `${md5}.gif`)
+    fs.ensureDirSync(path.dirname(appCache))
+    fs.writeFileSync(appCache, invalidCache)
+
+    const accountRoot = path.join(homeDir, 'account')
+    const wechatCache = path.join(accountRoot, 'cache', '2026-08', 'Emoticon', md5.slice(0, 2), md5)
+    fs.ensureDirSync(path.dirname(wechatCache))
+    fs.writeFileSync(wechatCache, invalidCache)
+
+    const fixture = await serve(validGif)
+    const client = { getAccountRoot: () => accountRoot } as unknown as Wcdb4Client
+    const result = await new StickerService(client).resolveSticker(fixture.url, md5)
+
+    expect(result.success, result.error).toBe(true)
+    expect(result.data).toBe(`data:image/gif;base64,${validGif.toString('base64')}`)
+    expect(fixture.requests()).toBe(1)
+    expect(fs.readFileSync(appCache)).toEqual(validGif)
+    expect(fs.readFileSync(wechatCache)).toEqual(invalidCache)
+  })
+
+  it('rejects an HTTP 200 response whose body is not an image', async () => {
+    const md5 = 'a'.repeat(32)
+    const fixture = await serve(Buffer.from('expired sticker response'))
+    const result = await new StickerService().resolveSticker(fixture.url, md5)
+
+    expect(result).toMatchObject({
+      success: false,
+      error: '表情包响应不是支持的图片格式'
+    })
+    expect(fixture.requests()).toBe(1)
+    expect(
+      fs.existsSync(path.join(homeDir, 'Documents', 'TraceMemo', 'Emojis', `${md5}.gif`))
+    ).toBe(false)
+  })
+})