fix: 图片密钥内存扫描通过子进程调用解决 task_for_pid 权限问题

Electron 进程缺少 com.apple.security.cs.debugger entitlement，
导致 ScanMemoryForImageKey 中的 task_for_pid 调用失败（kr=5）。

新增 image_scan_helper 子进程包装程序（与 xkey_helper 方案一致）：
- 新建 resources/image_scan_helper.c：dlopen libwx_key.dylib 并调用
  ScanMemoryForImageKey，通过 JSON stdout 返回结果
- 新建 resources/image_scan_entitlements.plist：包含 debugger 和
  allow-unsigned-executable-memory entitlements
- 编译为 universal binary（x86_64 + arm64）并 ad-hoc 签名
- 修改 keyServiceMac.ts _scanMemoryForAesKey()：优先 spawn
  image_scan_helper 子进程，失败时 fallback 到直接调 dylib

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

resources/image_scan_helper.c

@@ -0,0 +1,77 @@
+/*
+ * image_scan_helper - 轻量包装程序
+ * 加载 libwx_key.dylib 并调用 ScanMemoryForImageKey
+ * 用法: image_scan_helper <pid> <ciphertext_hex>
+ * 输出: JSON {"success":true,"aesKey":"..."} 或 {"success":false,"error":"..."}
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <dlfcn.h>
+#include <libgen.h>
+#include <mach-o/dyld.h>
+
+typedef const char* (*ScanMemoryForImageKeyFn)(int pid, const char* ciphertext);
+typedef void (*FreeStringFn)(const char* str);
+
+int main(int argc, char* argv[]) {
+    if (argc != 3) {
+        fprintf(stderr, "Usage: %s <pid> <ciphertext_hex>\n", argv[0]);
+        printf("{\"success\":false,\"error\":\"invalid arguments\"}\n");
+        return 1;
+    }
+
+    int pid = atoi(argv[1]);
+    const char* ciphertext_hex = argv[2];
+
+    if (pid <= 0) {
+        printf("{\"success\":false,\"error\":\"invalid pid\"}\n");
+        return 1;
+    }
+
+    /* 定位 dylib: 与自身同目录下的 libwx_key.dylib */
+    char exe_path[4096];
+    uint32_t size = sizeof(exe_path);
+    if (_NSGetExecutablePath(exe_path, &size) != 0) {
+        printf("{\"success\":false,\"error\":\"cannot get executable path\"}\n");
+        return 1;
+    }
+
+    char* dir = dirname(exe_path);
+    char dylib_path[4096];
+    snprintf(dylib_path, sizeof(dylib_path), "%s/libwx_key.dylib", dir);
+
+    void* handle = dlopen(dylib_path, RTLD_LAZY);
+    if (!handle) {
+        printf("{\"success\":false,\"error\":\"dlopen failed: %s\"}\n", dlerror());
+        return 1;
+    }
+
+    ScanMemoryForImageKeyFn scan_fn = (ScanMemoryForImageKeyFn)dlsym(handle, "ScanMemoryForImageKey");
+    if (!scan_fn) {
+        printf("{\"success\":false,\"error\":\"symbol not found: ScanMemoryForImageKey\"}\n");
+        dlclose(handle);
+        return 1;
+    }
+
+    FreeStringFn free_fn = (FreeStringFn)dlsym(handle, "FreeString");
+
+    fprintf(stderr, "[image_scan_helper] calling ScanMemoryForImageKey(pid=%d, ciphertext=%s)\n", pid, ciphertext_hex);
+
+    const char* result = scan_fn(pid, ciphertext_hex);
+
+    if (result && strlen(result) > 0) {
+        /* 检查是否是错误 */
+        if (strncmp(result, "ERROR", 5) == 0) {
+            printf("{\"success\":false,\"error\":\"%s\"}\n", result);
+        } else {
+            printf("{\"success\":true,\"aesKey\":\"%s\"}\n", result);
+        }
+        if (free_fn) free_fn(result);
+    } else {
+        printf("{\"success\":false,\"error\":\"no key found\"}\n");
+    }
+
+    dlclose(handle);
+    return 0;
+}