profile: bootstrap avatar URL on app boot

The appbar's <Avatar> reads avatarUrl from the localStorage profile
mirror. Without a global fetcher, that mirror only got populated when
the user navigated to /profile, so a fresh browser session showed
initials in the appbar until then.

- ProfileBootstrap component runs in root.tsx alongside
  LlmConfigBootstrap. On mount and on session change, fetches the
  arcadia profile and caches the resolved avatar URL.
- profile.tsx loadAccount now also persists the URL into localStorage
  on initial fetch (was in-memory only) so it survives reloads.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/profile.tsx

@@ -115,7 +115,17 @@ export default function ProfileRoute() {
           timezone: p.timezone ?? "",
         })
         const url = pickAvatarUrl(p)
-        if (url) setPrefs((d) => ({ ...d, avatarUrl: url }))
+        if (url) {
+          // Persist into localStorage so the appbar's useProfile() picks
+          // it up on next render — without this, the appbar avatar reverts
+          // to initials on every fresh browser session until the user
+          // re-uploads.
+          setPrefs((d) => {
+            const next = { ...d, avatarUrl: url }
+            saveProfile(next)
+            return next
+          })
+        }
       }
     } catch (err) {
       setAccountError(