feat(icons): propagate icon_custom through export / import / sync

Follow-up to 18fe13a: the custom-icon flag now survives round-trips instead
of resetting to fetched on a restore.

- Containers write icon_custom: manual export (doExport), auto-backup, sync
  snapshot; parseEntriesFromJSON carries it back (default 0 when absent, e.g.
  foreign CSV).
- encryptImportEntry ships it in the body (shared by bulk-import + sync
  create/update).
- Server create + bulk-import INSERTs read icon_custom
  (GetValue<Boolean>('icon_custom', False) → 0 default). An empty icon always
  forces icon_custom=0.

Now a custom icon stays custom after export→import and across sync — it won't
be silently reclassified as cache and clobbered by Re-fetch-all on the other
device.

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

js/app.sync.js

@@ -259,6 +259,7 @@ async function buildSyncSnapshot() {
             custom_fields:  plainCustom,
             attachments,
             icon_b64:       e.icon_b64 || '',
+            icon_custom:    e.icon_custom ? 1 : 0,
             created_at:     e.created_at,
             updated_at:     e.updated_at,
         });