unlock with PIN (#8977)

* add custom password to unlock settings

* If not set, use admin password; if set, use custom settings password.
* At least 4 characters.
* Set with gui or command line.

Signed-off-by: 21pages <sunboeasy@gmail.com>

* Update cn.rs

---------

Signed-off-by: 21pages <sunboeasy@gmail.com>
Co-authored-by: RustDesk <71636191+rustdesk@users.noreply.github.com>

src/flutter_ffi.rs

@@ -1617,6 +1617,14 @@ pub fn main_check_super_user_permission() -> bool {
     check_super_user_permission()
 }
 
+pub fn main_get_unlock_pin() -> SyncReturn<String> {
+    SyncReturn(get_unlock_pin())
+}
+
+pub fn main_set_unlock_pin(pin: String) -> SyncReturn<String> {
+    SyncReturn(set_unlock_pin(pin))
+}
+
 pub fn main_check_mouse_time() {
     check_mouse_time();
 }