feat(auth): password reset, forgot username, and admin user management for local accounts

- Add /forgot-password and /forgot-username page routes and templates
- Update login page label to "Username or Email" (both already accepted by backend)
- Add "Forgot password?" and "Forgot username?" links to login page
- Add POST /api/auth/forgot-username endpoint + send_forgot_username_email() utility
- Add admin endpoints: PATCH /local/{id}, POST /local/{id}/send-password-reset, POST /local/{id}/set-password
- Update admin_users.html with Edit, Password, and Reset action buttons + modals
- Add 23 tests; fix code review issues (import style, display_name clearing behaviour)
- Update docs/API.md and docs/UserGuide.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/admin_users.html

@@ -1122,7 +1122,7 @@ function adminUsersApp() {
           },
           body: JSON.stringify({
             email: this.editLocalUserModal.form.email || null,
-            display_name: this.editLocalUserModal.form.display_name || null,
+            display_name: this.editLocalUserModal.form.display_name,
             is_admin: this.editLocalUserModal.form.is_admin,
             is_active: this.editLocalUserModal.form.is_active,
           }),