fix(frontend): keep the widget link color off the check-in badge

sidebar.css paints .widget a with --color-ink-2 (specificity 0,1,1), which
out-specifies the .jb-* color group (0,1,0) the badge relies on for its white
ink. The user-card widget carries the .widget class, so the check-in bookmark
rendered dark-gray text on the orange gradient on widget pages. zibll sets no
link color on zib-widget anchors at all.

sidebar.css already restores .but colors for exactly this reason; extend that
restoration to .img-badge so the badge keeps its own --this-color. Rebuild the
homepage bundle and extend the badge contract test.

assets/css/frontend/sidebar.css

@@ -50,8 +50,9 @@
   color: var(--color-primary, #f04494);
 }
 
-/* .widget a 的链接色不得覆盖 .but 按钮自身配色（zibll 的 zib-widget 不设置链接色） */
-.widget a.but {
+/* .widget a 的链接色不得覆盖 .but / .img-badge 自身配色（zibll 的 zib-widget 不设置链接色） */
+.widget a.but,
+.widget a.img-badge {
   color: var(--this-color, #fff);
 }