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/BottomNav.svelte

@@ -1,7 +1,8 @@
 <script>
   // Mobile-only primary navigation. Today = the brief, Browse = mood/topic
-  // discovery, You = personal controls (Boundaries, History).
-  let { active = 'today', onToday, onBrowse, onYou } = $props();
+  // discovery, You = account + personal controls (shows the user's avatar in).
+  import Avatar from './Avatar.svelte';
+  let { active = 'today', onToday, onBrowse, onYou, user = null } = $props();
 </script>
 
 <nav class="bottomnav" aria-label="Primary">
@@ -14,7 +15,11 @@
     <span>Browse</span>
   </button>
   <button class:active={active === 'you'} onclick={onYou}>
-    <svg viewBox="0 0 24 24" aria-hidden="true"><circle cx="12" cy="8.5" r="3.6" fill="none" stroke="currentColor" stroke-width="1.8" /><path d="M5 20c0-3.6 3.1-5.5 7-5.5s7 1.9 7 5.5" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" /></svg>
+    {#if user}
+      <Avatar {user} size={23} />
+    {:else}
+      <svg viewBox="0 0 24 24" aria-hidden="true"><circle cx="12" cy="8.5" r="3.6" fill="none" stroke="currentColor" stroke-width="1.8" /><path d="M5 20c0-3.6 3.1-5.5 7-5.5s7 1.9 7 5.5" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" /></svg>
+    {/if}
     <span>You</span>
   </button>
 </nav>