Add admin-assisted password reset

There's no email infrastructure in this app, so a self-service
"forgot password" flow isn't feasible yet. Adds a "Reset Password"
button per user in the admin panel instead: generates a random
temporary password (shown once, for the admin to relay out-of-band),
overwrites the user's password hash, and signs them out of every
existing session so a stolen session can't outlive the reset.

Verified live: old password rejected after reset, new temporary
password logs in successfully.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/syncService.js

@@ -171,6 +171,11 @@ export async function adminDeleteUser(id) {
   return request('DELETE', `/admin/users/${id}`);
 }
 
+/** Sets a random temporary password for a user and signs them out everywhere. Returns { temporaryPassword }. */
+export async function adminResetPassword(id) {
+  return request('POST', `/admin/users/${id}/reset-password`);
+}
+
 export async function adminListProjects() {
   return request('GET', '/admin/projects');
 }