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

apps/docs/content/(01-linear-structures)/two-pointers/125-valid-palindrome.mdx

@@ -2,13 +2,15 @@
 title: '125. Valid Palindrome'
 description: A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers
 icon: dot
+topics:
+  - { name: "Two Pointers", slug: "two-pointers" }
+  - { name: "String", slug: "string" }
+video: "jJXJ16kPFWg"
 sidebar:
   label: 'Valid Palindrome'
   badge: 'Easy'
 ---
 
-<Badge variant="accent">Two Pointers</Badge>
-
 ### Example 1:
 - Input: `s = "A man, a plan, a canal: Panama"`
 - Output: `true`
@@ -64,7 +66,3 @@ var isPalindrome = function(s) {
   return true;
 };
 ```
-
-## Explanation
-
-<YouTube id="jJXJ16kPFWg" title="Valid Palindrome - Leetcode 125 - Python" />