💄refactor: enhance EditUser and AddUser form validation & UX

Changes in `web/src/pages/User/EditUser.js`:
• Added `rules` to
  – `Form.Select group`: now required with error “Please select group”.
  – `Form.InputNumber quota`: now required with error “Please enter quota”.
• Added `step={500000}` to quota `InputNumber` for quicker numeric input.
• Replaced invalid `readonly` with React-correct `readOnly`, and added descriptive placeholders for all binding-info fields (GitHub/OIDC/WeChat/Email/Telegram).
• Removed unused `downloadTextAsFile` import.

These updates tighten form validation, improve data entry ergonomics, and restore clear read-only indicators for third-party bindings.

web/src/components/settings/PersonalSetting.js

@@ -967,7 +967,7 @@ const PersonalSetting = () => {
                                 {systemToken && (
                                   <div className="mt-3">
                                     <Input
-                                      readOnly
+                                      readonly
                                       value={systemToken}
                                       onClick={handleSystemTokenClick}
                                       size="large"