feat: populate note files with problem descriptions and code stubs

Add populate-notes.mjs that fetches problem descriptions and
Python/C++ code stubs from LeetCode's GraphQL API. Populated
all 197 NeetCode 150 note files with:
- Problem description (examples, constraints)
- Python code stub (function signature)
- C++ code stub (function signature + includes)

API responses cached in leetcode/.cache/leetcode/ for instant re-runs.

org/study_deck_02/dsa/math-geometry/0202-happy-number.org

@@ -1,18 +1,63 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0202. Happy Number :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0202. Happy Number][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0202. Happy Number][0202. Happy Number]]
 :END:
 
+Write an algorithm to determine if a number ~n~ is happy.
+
+A *happy number* is a number defined by the following process:
+
+ - Starting with any positive integer, replace the number by the sum of the squares of its digits.
+
+ - Repeat the process until the number equals 1 (where it will stay), or it *loops endlessly in a cycle* which does not include 1.
+
+ - Those numbers for which this process *ends in 1* are happy.
+
+Return ~true~ /if/ ~n~ /is a happy number, and/ ~false~ /if not/.
+
+*Example 1:*
+
+
+#+begin_src
+Input: n = 19
+Output: true
+Explanation:
+12 + 92 = 82
+82 + 22 = 68
+62 + 82 = 100
+12 + 02 + 02 = 1
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: n = 2
+Output: false
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= n <= 2^{31} - 1~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def isHappy(self, n: int) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    bool isHappy(int n) {
+        
+    }
+};
 #+end_src