refactor: 移除微信 3.0 解密路径, 统一走 4.0 WCDB

- wechat-db.ts 删除 SQLCipher 回退、connectDb、getUser、tryOpenWechat4 等 3.0 专属逻辑
- image-decrypt-service.ts 删除 decryptDatV3 与 version === 0 分支
- index.ts 去除 wcdb4Client 冗余空值检查
- 移除 better-sqlite3-multiple-ciphers 依赖
- README 版本前置改为 4.0+

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/main/image-decrypt-service.ts

@@ -241,15 +241,11 @@ export class ImageDecryptService {
       )
 
       let decrypted: Buffer
-      if (version === 0) {
-        console.log('[ImageDecrypt] using V3 (XOR only)')
-        decrypted = this.decryptDatV3(datPath)
-      } else if (version === 1) {
+      if (version === 1) {
         console.log('[ImageDecrypt] using V1 (default AES key)')
         const key = Buffer.from(this.defaultV1AesKey, 'ascii')
         decrypted = this.decryptDatV4(datPath, key)
-      } else {
-        // version === 2
+      } else if (version === 2) {
         console.log('[ImageDecrypt] using V2 (user AES key)')
         if (!this.aesKey) {
           console.log('[ImageDecrypt] no AES key configured')
@@ -257,6 +253,9 @@ export class ImageDecryptService {
         }
         const key = Buffer.from(this.aesKey, 'ascii').slice(0, 16)
         decrypted = this.decryptDatV4(datPath, key)
+      } else {
+        console.log('[ImageDecrypt] unsupported dat version:', version)
+        return null
       }
 
       return decrypted
@@ -310,18 +309,6 @@ export class ImageDecryptService {
     return 0
   }
 
-  /**
-   * V3 解密 - 仅 XOR
-   */
-  private decryptDatV3(inputPath: string): Buffer {
-    const data = readFileSync(inputPath)
-    const out = Buffer.alloc(data.length)
-    for (let i = 0; i < data.length; i += 1) {
-      out[i] = data[i] ^ this.xorKey
-    }
-    return out
-  }
-
   /**
    * V4 解密 - AES + XOR
    */