feat(sync): include avatar in sync snapshot + auto-backup (multi-device)

The profile picture (users.avatar_b64, cosmetic/unencrypted) travelled only
in the manual export. Now it's also in buildSyncSnapshot and the auto-backup
container, so a new device / a restore picks it up.

- Restore is ADDITIVE (mirrors the import path): applyRemoteSnapshot adopts
  remote.avatar_b64 only when the local device has no avatar — never clobbers
  a locally-set picture. No per-avatar timestamp to arbitrate, so changing an
  existing avatar doesn't propagate (cosmetic, accepted).
- +2 merge tests (adopt-when-empty, don't-clobber-when-set). 65/65 green.
- Server /avatar endpoint unchanged (already accepts {avatar_b64}).

Closes the avatar item of CODE_AUDIT §4.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.backup.js

@@ -146,6 +146,9 @@ async function runAutoBackupNow(silent) {
                            color: f.color || '',
                            icon:  f.icon  || '',
                        })),
+            // Same as the manual export container — keep the avatar so a
+            // restore from an auto-backup brings the profile picture back.
+            avatar_b64: state.avatarDataUri || '',
             entries: [],
         };
         let _bkDone = 0;