Parent controls for children, name-tile sign-in, restricted Learn search, simple mode

- Sign-in page shows name tiles on remembered devices; tap a name, then PIN or passkey
- Music and Movies are separate on the server; Plex can be locked to a child's managed profile
- Activity log, weekly summary and alerts to parents; per-area and weekend limits
- Requests from children (Ask button) answered in Settings; bonus screen time from rewards
- Restricted Learn search (Simple English Wikipedia, blocked words) replaces web search for children
- Simple mode (big-button home) for young children or grandparents
- Child usage kept in memory instead of read from disk on every request
- Tests and README updated

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/webauthn.js

@@ -22,8 +22,8 @@ async function passkeyRegister(setupCode, name, inviteCode) {
     clientExtensionResults: cred.getClientExtensionResults(), authenticatorAttachment: cred.authenticatorAttachment,
   } });
 }
-async function passkeyLogin() {
-  const { cid, options } = await authPost('/api/auth/login/options');
+async function passkeyLogin(uid) {
+  const { cid, options } = await authPost('/api/auth/login/options', uid ? { uid } : undefined);
   options.challenge = b64u.dec(options.challenge);
   (options.allowCredentials || []).forEach(c => { c.id = b64u.dec(c.id); });
   const a = await navigator.credentials.get({ publicKey: options });