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

islands/decks/arrays-and-strings.md

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+## Arrays & Strings
+
+Ordered collections. Arrays are **mutable**, strings are **immutable** — every "edit" to a string builds a new one.
+
+---
+
+## Cost model
+
+| Operation | Array | String |
+| --- | --- | --- |
+| Append / pop at end | O(1)* | O(n) |
+| Insert / delete in middle | O(n) | O(n) |
+| Random access | O(1) | O(1) |
+| Membership check | O(n) | O(n) |
+
+*amortized — occasional resize, constant on average.
+
+---
+
+## Recognition
+
+- Contiguous data + index arithmetic → array patterns apply
+- Building a string piece by piece → collect parts, join once
+- "In-place" requirement → think swap / overwrite, not rebuild
+
+---
+
+## String building
+
+```js
+// O(n^2): each += copies the whole string
+let s = "";
+for (const ch of parts) s += ch;
+
+// O(n): buffer then join
+const buf = [];
+for (const ch of parts) buf.push(ch);
+const out = buf.join("");
+```
+
+---
+
+## Recall
+
+- Why is append amortized O(1) and not plain O(1)?
+- What does immutability cost when you "modify" one character?
+- Which patterns sit on top of arrays? (two pointers, sliding window, prefix sum)
+
+---
+
+## Drill
+
+- 1480 Running Sum of 1d Array
+- 217 Contains Duplicate
+- 189 Rotate Array
+- 977 Squares of a Sorted Array