fix(web): use relative path for high-score API endpoint

Change the high-score endpoint URL in storage.js from "/api/highscores" to
"./api/highscores". When serving the web build from a subpath rather than the
domain root, an absolute path sends fetch requests to the domain root instead
of the nested application path. Using a relative URL ensures requests resolve
relative to the active document path while still working when hosted at root.

Test Plan:
- `node --check tdkpin-rs/web/storage.js` -- passed
- `npx prettier --check tdkpin-rs/web/storage.js` -- passed
- `just test` -- passed (138 game tests, 3 highscore-server tests)
- `just clippy` -- passed
- `git diff --cached --check` -- passed

tdkpin-rs/web/storage.js

@@ -4,7 +4,7 @@
   const storageKey = "tdkpin.save.v1";
   const encoder = new TextEncoder();
   const decoder = new TextDecoder();
-  const highScoreApi = "/api/highscores";
+  const highScoreApi = "./api/highscores";
   const highScoreRetryInitialDelayMs = 250;
   const highScoreRetryMaxDelayMs = 30_000;
   let lastRevision = 0;