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

islands/decks/sliding-window.md

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+## Sliding Window
+
+Two pointers **plus incremental state** between them. Never recompute the window from scratch.
+
+---
+
+## Recognition
+
+- "Longest / shortest **contiguous** subarray or substring such that…"
+- Constraint is monotone: growing the window can only make it worse
+- Fixed window size k → same idea, both ends move together
+
+---
+
+## Template — variable window (LC 3)
+
+```js
+function lengthOfLongestSubstring(s) {
+  const seen = new Set();
+  let best = 0;
+  let l = 0;
+  for (let r = 0; r < s.length; r++) {
+    while (seen.has(s[r])) {   // shrink until valid
+      seen.delete(s[l]);
+      l++;
+    }
+    seen.add(s[r]);
+    best = Math.max(best, r - l + 1);
+  }
+  return best;
+}
+```
+
+Grow with `r`, shrink with `l` only while invalid.
+
+---
+
+## Why it's O(n)
+
+- `r` moves n times, `l` moves at most n times
+- Each element enters and leaves the window **once**
+- The inner `while` is amortized, not nested work
+
+---
+
+## Pitfalls
+
+- State must update in O(1) on both add and remove
+- "Shortest window" flips the loop: shrink while **valid**, record before breaking
+- Doesn't apply when the constraint isn't monotone (mixed signs → prefix sum)
+
+---
+
+## Recall
+
+- LC 121 Best Time to Buy/Sell: what is the "window state"? (min so far)
+- LC 76 Minimum Window: what makes a window "valid"?
+- When must you fall back to prefix sums instead?
+
+---
+
+## Drill
+
+- 121 Best Time to Buy and Sell Stock
+- 3 Longest Substring Without Repeating Characters
+- 424 Longest Repeating Character Replacement
+- 567 Permutation in String · 76 Minimum Window Substring
+- 209 Minimum Size Subarray Sum