chore(assets): migrate zibll img resources to assets/img

Copy Zibll's widget/visual reference assets into assets/img for
1:1 replication (Phase 5 resource migration):
- slider-bg.jpg, slider-layer-1.png, slider-layer-2.png
- user_t.jpg, avatar-default.png
- null*.svg (null, null-2, null-cap, null-love, null-money,
  null-order, null-post, null-search, null-set, null-user)
- thumbnail*.svg (thumbnail, thumbnail-lg, thumbnail-sm,
  thumbnail-null)
- medal/ (31 medal-*.svg badges)

These are reference placeholders to be progressively replaced;
no CSF framework ported. user-level-1..10.png already present.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/img/null-set.svg

@@ -0,0 +1 @@
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