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/routes/layout.svelte.spec.ts

@@ -16,9 +16,9 @@ const tick = () => new Promise((r) => setTimeout(r, 0));
 const makeData = (overrides = {}) => ({
 	user: {
 		id: '1',
-		username: 'max',
 		firstName: 'Max',
 		lastName: 'Müller',
+		email: 'max@example.com',
 		groups: [],
 		enabled: true,
 		createdAt: ''
@@ -39,7 +39,7 @@ describe('Layout – user avatar button', () => {
 	it('shows fallback icon button when names are not set', async () => {
 		render(Layout, {
 			data: makeData({
-				user: { id: '1', username: 'x', groups: [], enabled: true, createdAt: '' }
+				user: { id: '1', email: 'fallback@example.com', groups: [], enabled: true, createdAt: '' }
 			}),
 			children: emptySnippet
 		});