feat(auth): migrate frontend from username to email-only authentication

- Login page: email input replaces username field (type=email, name=email)
- Login server action: reads email, uses i18n error for missing credentials
- AccountSection: email input (type=email) replaces username text field
- New user server action: sends email as required field, drops username
- UsersListPanel: displays and searches by email instead of username
- Admin edit user page: heading and delete confirm use email
- Profile page: fullName fallback uses email, drops @username display
- app.d.ts: email required on User, username removed
- Generated API types: AppUser.email required, username removed; CreateUserRequest.email required, username removed
- i18n: login_label_email, login_error_missing_credentials, admin_col_login updated (de/en/es)
- errors.ts: MISSING_CREDENTIALS → login_error_missing_credentials

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

frontend/src/lib/generated/api.ts

@@ -1253,13 +1253,12 @@ export interface components {
         AppUser: {
             /** Format: uuid */
             id: string;
-            username: string;
             password?: string;
             firstName?: string;
             lastName?: string;
             /** Format: date */
             birthDate?: string;
-            email?: string;
+            email: string;
             contact?: string;
             enabled: boolean;
             notifyOnReply: boolean;
@@ -1406,8 +1405,7 @@ export interface components {
             blockIds?: string[];
         };
         CreateUserRequest: {
-            username?: string;
-            email?: string;
+            email: string;
             initialPassword?: string;
             groupIds?: string[];
             firstName?: string;