refactor: complete JS to TypeScript migration for spreadsheet engine

- Convert all 10 source files from JS to TypeScript
- Create src/types.ts with 18 interfaces/types (CellStyle, CellBorder, CellValue, etc.)
- Add global Window type extensions for WolfTable
- Type Canvas2d class with index signature for dynamic proxy methods
- Type Area class with RowGetter/ColGetter/RowInfo/ColInfo
- Type Range class: fix within() bug (startCode → startCol)
- Type Range.empty: fix getter calling rows() as function instead of property
- Type Viewport with TableContext interface
- Type Table class with all 20+ properties
- Type throttle as generic function
- Type event handlers with MouseEvent casts
- Fix alphabet.ts: parseInt → Math.floor for column index calculation
- Use Math.min/Math.max in Range.union() instead of ternary
- Add JSDoc documentation to all public APIs
- Update tsconfig: allowJs=false

Author: refactor-bot

src/alphabet.js

@@ -0,0 +1,61 @@
+/** Column alphabet letters. */
+const alphabets: string[] = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];
+
+/** Convert a zero-based column index to a column letter string (e.g., 0 → 'A', 26 → 'AA'). */
+export function stringAt(index: number): string {
+  let str = '';
+  let cindex = index;
+  while (cindex >= alphabets.length) {
+    cindex /= alphabets.length;
+    cindex -= 1;
+    str += alphabets[Math.floor(cindex) % alphabets.length];
+  }
+  const last = index % alphabets.length;
+  str += alphabets[last];
+  return str;
+}
+
+/** Convert a column letter string to a zero-based index (e.g., 'A' → 0, 'AA' → 26). */
+export function indexAt(str: string): number {
+  let ret = 0;
+  for (let i = 0; i < str.length - 1; i += 1) {
+    const cindex = str.charCodeAt(i) - 65;
+    const exponent = str.length - 1 - i;
+    ret += (alphabets.length ** exponent) + (alphabets.length * cindex);
+  }
+  ret += str.charCodeAt(str.length - 1) - 65;
+  return ret;
+}
+
+/** Convert a cell reference (e.g., 'B10') to [colIndex, rowIndex]. */
+export function expr2xy(src: string): [number, number] {
+  let x = '';
+  let y = '';
+  for (let i = 0; i < src.length; i += 1) {
+    if (src.charAt(i) >= '0' && src.charAt(i) <= '9') {
+      y += src.charAt(i);
+    } else {
+      x += src.charAt(i).toUpperCase();
+    }
+  }
+  return [indexAt(x), parseInt(y, 10) - 1];
+}
+
+/** Convert [colIndex, rowIndex] to a cell reference (e.g., [1, 9] → 'B10'). */
+export function xy2expr(x: number, y: number): string {
+  return `${stringAt(x)}${y + 1}`;
+}
+
+/** Offset a cell reference by (xn, yn). */
+export function expr2expr(src: string, xn: number, yn: number): string {
+  const [x, y] = expr2xy(src);
+  return xy2expr(x + xn, y + yn);
+}
+
+export default {
+  stringAt,
+  indexAt,
+  expr2xy,
+  xy2expr,
+  expr2expr,
+};