docs(docs): add Approach and Explanation sections with mermaid diagrams to problem pages

apps/docs/content/(hash-table)/387-first-unique-character-in-a-string.mdx

@@ -26,6 +26,18 @@ sidebar:
 - `1 <= s.length <= 10^5`
 - `s` consists of only lowercase English letters.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["firstUniqChar(s)"]) --> F["Pass 1 — freq: count every char of s"]
+  F --> L{"Pass 2 — more i in 0..len-1?"}
+  L -- no --> E(["return -1"])
+  L -- yes --> Q{"freq[s[i]] == 1?"}
+  Q -- yes --> R(["return i — leftmost wins because the scan is in order"])
+  Q -- no --> L
+```
+
 ## Solution
 
 ```js
@@ -45,3 +57,7 @@ var firstUniqChar = function (s) {
   return -1;
 };
 ```
+
+## Explanation
+
+<YouTube id="rBENYgWy3xU" title="First Unique Character in a String - Leetcode 387 - Python" />