feat(islands): add StudyDeck component and markdown decks for algorithms

islands/decks/two-pointers.md

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+## Two Pointers
+
+Walk two indices instead of trying all pairs — **order lets you prune O(n²) → O(n)**.
+
+---
+
+## Recognition
+
+- Sorted array + pair/target condition → converge from both ends
+- Palindrome / symmetric check → ends inward
+- In-place partition or dedupe → slow writer + fast reader
+- Two sequences compared → one pointer each
+
+---
+
+## Template — converging (LC 167)
+
+```js
+function twoSumSorted(nums, target) {
+  let l = 0;
+  let r = nums.length - 1;
+  while (l < r) {
+    const sum = nums[l] + nums[r];
+    if (sum === target) return [l + 1, r + 1];
+    if (sum < target) l++; // need a bigger sum
+    else r--;              // need a smaller sum
+  }
+}
+```
+
+Each step discards every pair using the abandoned index — that's the proof.
+
+---
+
+## Template — fast/slow writer (LC 977 idea)
+
+```js
+function sortedSquares(nums) {
+  const out = new Array(nums.length);
+  let l = 0;
+  let r = nums.length - 1;
+  for (let i = nums.length - 1; i >= 0; i--) {
+    const a = nums[l] * nums[l];
+    const b = nums[r] * nums[r];
+    if (a > b) { out[i] = a; l++; }
+    else       { out[i] = b; r--; }
+  }
+  return out;
+}
+```
+
+Largest squares live at the **edges** — fill the output backwards.
+
+---
+
+## Pitfalls
+
+- Moving the wrong pointer breaks the pruning argument
+- Duplicates in 3Sum: skip repeats after each fixed element
+- `l < r` vs `l <= r`: do the pointers meet or cross?
+
+---
+
+## Recall
+
+- Container With Most Water: why move the *shorter* wall?
+- 3Sum = sort + fix one + which pattern inside?
+- Valid Palindrome: what do you do with non-alphanumerics?
+
+---
+
+## Drill
+
+- 125 Valid Palindrome · 167 Two Sum II
+- 15 3Sum · 11 Container With Most Water
+- 42 Trapping Rain Water · 392 Is Subsequence
+- 977 Squares of a Sorted Array · 80 Remove Duplicates II