docs(docs): add ProblemBadges component, layout slot, and enrich pages with topics/video frontmatter, plus guide placeholders and topic‑landing map.

apps/docs/content/(05-decision-space)/backtracking/78-subsets.mdx

@@ -2,13 +2,15 @@
 title: '78. Subsets'
 description: Given an integer array nums of unique elements, return all possible subsets (the power set)
 icon: dot
+topics:
+  - { name: "Array", slug: "array" }
+  - { name: "Backtracking", slug: "backtracking" }
+  - { name: "Bit Manipulation", slug: "bit-manipulation" }
 sidebar:
   label: 'Subsets'
   badge: 'Medium'
 ---
 
-<Badge variant="accent">Array</Badge>
-
 ### Example 1:
 - Input: `nums = [1,2,3]`
 - Output: `[[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]`