User avatar (Google picture), avatar in mobile You tab, /account page

- Capture the Google profile picture (picture claim) into users.avatar_url; an
  Avatar component shows it, falling back to the initial. Used in the desktop
  header and the mobile "You" tab (which now shows the user when signed in).
- Move account/settings to its own route /account (robust + scrolls to top),
  reached by the desktop avatar and the mobile You tab; drop the inline "You"
  sheet. AccountPanel gains a Sign out action; the page links to Saved/History/
  Boundaries via home intent params (?view= / ?open=).
- db: users.avatar_url (schema + idempotent migration). 118 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/Header.svelte

@@ -1,6 +1,6 @@
 <script>
+  import Avatar from './Avatar.svelte';
   let { onBoundaries, onHistory, onaccount, user = null, filtersOn = false } = $props();
-  let initial = $derived((user?.display_name || user?.email || '?').trim()[0].toUpperCase());
 </script>
 
 <header class="appbar">
@@ -23,7 +23,7 @@
       </button>
       {#if user}
         <button class="acct" onclick={onaccount} title={user.email} aria-label="Your account">
-          <span class="avatar">{initial}</span>
+          <Avatar {user} size={30} />
         </button>
       {:else}
         <button class="signin" onclick={onaccount}>Sign in</button>
@@ -59,13 +59,7 @@
   .utils button.on { color: var(--accent-deep); }
   .utils .signin { border-color: var(--line); color: var(--accent-deep); }
   .utils .signin:hover { background: var(--accent-soft); }
-  .acct { padding: 4px; }
-  .avatar {
-    display: inline-flex; align-items: center; justify-content: center;
-    width: 30px; height: 30px; border-radius: 999px;
-    background: var(--accent); color: #fff; font-weight: 600; font-size: 0.8rem;
-    font-family: var(--label);
-  }
+  .acct { padding: 4px; display: inline-flex; }
 
   /* On phones the utilities live in the bottom tab bar ("You") instead. */
   @media (max-width: 720px) {