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/messages/en.json

@@ -53,8 +53,9 @@
 	"form_placeholder_archive_location": "e.g. Cabinet 3, Folder B",
 	"form_helper_archive_location": "Where is the original document stored?",
 	"login_heading": "Sign in",
-	"login_label_username": "Username",
+	"login_label_email": "Email",
 	"login_label_password": "Password",
+	"login_error_missing_credentials": "Please enter your email address and password.",
 	"login_btn_submit": "Sign in",
 	"docs_search_placeholder": "Search title, people, tags…",
 	"docs_sort_label": "Sort",
@@ -164,7 +165,7 @@
 	"admin_tab_groups": "Groups",
 	"admin_tab_tags": "Tags",
 	"admin_section_users": "User management",
-	"admin_col_login": "Login",
+	"admin_col_login": "Email",
 	"admin_col_groups": "Groups",
 	"admin_col_password": "Password",
 	"admin_multiselect_hint": "Ctrl+Click to select",