Add self-service "change my password" option in Account Settings

New section in Settings: current password + new password + confirm,
verified against the existing hash before accepting. On success it
signs out every other session for that account (in case one was
compromised) but keeps the current session logged in, so changing
your password doesn't immediately kick you back to the login screen.

Renamed the underlying db.js function (adminSetPassword ->
setUserPassword) since it's now shared by both this and the existing
admin-assisted reset.

Verified live: old password rejected after change, new password
works, current session stayed logged in throughout.

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

src/syncService.js

@@ -140,6 +140,11 @@ export async function updateProfile({ podcastName }) {
   return request('PATCH', '/auth/profile', { podcastName });
 }
 
+/** Self-service password change. Requires the current password; signs out every other session. */
+export async function changePassword(currentPassword, newPassword) {
+  return request('POST', '/auth/change-password', { currentPassword, newPassword });
+}
+
 // ---------------------------------------------------------------------------
 // Two-factor auth setup (Account Settings page)
 // ---------------------------------------------------------------------------