fix: 安全漏洞修复 — CLI密码泄露 + margin上限 + 路径检查 + CSP/限速

CRITICAL:
- CLI --password 未提供时自动从 QRGEN_WIFI_PASSWORD 环境变量读取
  避免密码出现在 ps aux / /proc/pid/cmdline 进程列表中

HIGH:
- unsafe from_utf8_unchecked SAFETY 注释增强, 详细解释为何有效
- CLI --margin 添加 value_parser range(0..=100), 防止 u8=255 导致内存爆炸
- Web CORS permissive 添加注释说明公开 API 设计理由
- GUI FS 权限收紧: 写权限从 /c/Users/33644/** 限制为 Downloads/Documents/Desktop/AppData

MEDIUM:
- 新增 QrError::InvalidEcLevel(String), 不再复用 InvalidVersion(0)
- --logo/--batch/--output_dir 均添加 check_path() 路径遍历检查
- Web 添加 tower::limit::ConcurrencyLimitLayer(10) 并发限制防 CPU 耗尽
- decode_image 添加 4096x4096 图片尺寸上限防解压炸弹

LOW:
- Web 添加 Content-Security-Policy 响应头

gui/gen/schemas/capabilities.json

@@ -1 +1 @@
-{"default":{"identifier":"default","description":"QRGen 默认权限","local":true,"windows":["main"],"permissions":["core:default","store:default","dialog:default","clipboard-manager:default",{"identifier":"fs:allow-write-file","allow":[{"path":"$HOME/**"}]},{"identifier":"fs:allow-read-file","allow":[{"path":"$HOME/**"},{"path":"$TEMP/**"}]}]}}
+{"default":{"identifier":"default","description":"QRGen 默认权限 — 最小权限原则","local":true,"windows":["main"],"permissions":["core:default","store:default","dialog:default","clipboard-manager:default",{"identifier":"fs:allow-write-file","allow":[{"path":"$HOME/Downloads/**"},{"path":"$HOME/Documents/**"},{"path":"$HOME/Desktop/**"},{"path":"$APPDATA/**"}]},{"identifier":"fs:allow-read-file","allow":[{"path":"$HOME/Downloads/**"},{"path":"$HOME/Documents/**"},{"path":"$HOME/Desktop/**"},{"path":"$HOME/Pictures/**"},{"path":"$APPDATA/**"},{"path":"$TEMP/**"}]}]}}