feat(user-models): fall back to category icon when no brand logo matches

src/assets/model_icons/stt.svg

@@ -0,0 +1,18 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none">
+  <defs>
+    <linearGradient id="stt-fill-grad" x1="12" y1="3" x2="12" y2="14" gradientUnits="userSpaceOnUse">
+      <stop offset="0%" stop-color="#40a9ff" stop-opacity="0.2"/>
+      <stop offset="100%" stop-color="#1677ff" stop-opacity="0.05"/>
+    </linearGradient>
+    <linearGradient id="stt-stroke-grad" x1="12" y1="3" x2="12" y2="21" gradientUnits="userSpaceOnUse">
+      <stop offset="0%" stop-color="#40a9ff"/>
+      <stop offset="100%" stop-color="#1677ff"/>
+    </linearGradient>
+  </defs>
+  <!-- 麦克风核心体：精装通透填充 -->
+  <rect x="8.5" y="3" width="7" height="11" rx="3.5" fill="url(#stt-fill-grad)" stroke="url(#stt-stroke-grad)" stroke-width="1.8" />
+  <!-- 内部音膜立体结构线（细化为点状） -->
+  <line x1="10" y1="8" x2="14" y2="8" stroke="url(#stt-stroke-grad)" stroke-width="1" stroke-dasharray="1 2"/>
+  <!-- 悬挂外托架与底座（全部改为高级圆角） -->
+  <path d="M5 10a7 7 0 0 0 14 0M12 17v4M8 21h8" stroke="url(#stt-stroke-grad)" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>