refactor: purge remaining TypeScript residue

After the JS+JSDoc conversion, some TS-flavored bits lingered. Removed:

- // @ts-check directives (TS-specific pragma)
- JSDoc annotations referencing TS-defined types: import('next').NextConfig,
  React.MutableRefObject, React.Dispatch, React.SetStateAction
- jsconfig.json (TS-server-flavored config; only kept it for the @/* alias)

@/* imports replaced with relative paths so jsconfig is no longer needed.
Remaining JSDoc is plain @param / @returns — vanilla JS, no TS dependency.

Build, lint, dev profiles unchanged.

docs/code-standards.md

@@ -125,8 +125,8 @@ function randomANumberInRow(weights) {
 ```js
 import { useCallback, useState } from "react";
 import Link from "next/link";
-import PlayerBoard from "@/components/player-board";
-import { generateGrid } from "@/lib/game-logic";
+import PlayerBoard from "../components/player-board";
+import { generateGrid } from "../lib/game-logic";
 ```
 
 ## Testing (Not Currently Implemented)