refactor(components): add appendix tag icons and drop list badge

apps/docs/components/BacklinksList.astro

@@ -1,9 +1,10 @@
 ---
 /**
  * The `PageFooter` slot: an Obsidian-style backlinks list under a topic landing
- * or guide page — every problem page that points at it, as `<num> <name>`
- * rows in LeetCode-number order. Pages nothing points at (problem pages, the
- * overview, the changelog) render nothing.
+ * page — every problem page that points at it, as `<num> <name>` rows in
+ * LeetCode-number order. Pages nothing points at (problem pages, the
+ * overview) render nothing; a tag no topic teaches lists its problems on its
+ * Appendix card instead (`pages/appendix.astro`), from the same answer.
  *
  * A problem points at a page when either
  * - one of its topic badges links there — the same `tagTarget()` answer