fix(web): back off high-score retries

The browser previously retried every failed shared high-score submission on the
50 ms polling interval. With server-side rate limiting and database load
shedding, immediate 429 and 503 responses could create a tight retry loop.
Add a bounded exponential delay with jitter, honor Retry-After on transient
responses, and reset the delay after a successful submission. Keep the pending
revision retryable without allowing overlapping requests.

Test Plan:
- `node --check tdkpin-rs/web/storage.js` -- passed
- `prettier --check tdkpin-rs/web/storage.js` -- passed
- Node VM retry timing harness covering Retry-After, backoff, and reset -- passed
- `git diff --cached --check` -- passed

tdkpin-rs/web/storage.js

@@ -5,11 +5,15 @@
   const encoder = new TextEncoder();
   const decoder = new TextDecoder();
   const highScoreApi = "/api/highscores";
+  const highScoreRetryInitialDelayMs = 250;
+  const highScoreRetryMaxDelayMs = 30_000;
   let lastRevision = 0;
   let lastHighScoreRevision = 0;
   let highScoreRequestInFlight = false;
   let highScoreGeneration = 0;
   let highScoreSubmissionWarningShown = false;
+  let highScoreRetryDelayMs = highScoreRetryInitialDelayMs;
+  let highScoreRetryAt = 0;
 
   function browserStorage() {
     try {
@@ -86,10 +90,50 @@
     }
   }
 
+  function retryAfterDelay(response) {
+    if (response.status !== 429 && response.status !== 503) {
+      return null;
+    }
+    const value = response.headers.get("Retry-After")?.trim();
+    if (!value) {
+      return null;
+    }
+
+    const seconds = Number(value);
+    if (Number.isFinite(seconds) && seconds >= 0) {
+      const delay = seconds * 1000;
+      return Number.isFinite(delay) ? delay : null;
+    }
+
+    const timestamp = Date.parse(value);
+    return Number.isFinite(timestamp)
+      ? Math.max(0, timestamp - Date.now())
+      : null;
+  }
+
+  function scheduleHighScoreRetry(retryAfterMs) {
+    const jitteredDelay = highScoreRetryDelayMs * (0.5 + Math.random());
+    const delay =
+      retryAfterMs === null ? jitteredDelay : retryAfterMs + jitteredDelay;
+    highScoreRetryAt = Date.now() + delay;
+    highScoreRetryDelayMs = Math.min(
+      highScoreRetryDelayMs * 2,
+      highScoreRetryMaxDelayMs,
+    );
+  }
+
+  function resetHighScoreRetry() {
+    highScoreRetryDelayMs = highScoreRetryInitialDelayMs;
+    highScoreRetryAt = 0;
+  }
+
   async function flushHighScoreSubmission() {
     if (highScoreRequestInFlight) {
       return;
     }
+    if (Date.now() < highScoreRetryAt) {
+      return;
+    }
     const revision = wasm_exports.tdkpin_browser_high_score_revision();
     if (revision === lastHighScoreRevision) {
       return;
@@ -102,6 +146,7 @@
     }
 
     highScoreRequestInFlight = true;
+    let retryAfterMs = null;
     try {
       const response = await fetch(highScoreApi, {
         method: "POST",
@@ -110,6 +155,7 @@
         keepalive: true,
       });
       if (!response.ok) {
+        retryAfterMs = retryAfterDelay(response);
         throw new Error(`high-score submission failed (${response.status})`);
       }
       const json = await response.text();
@@ -118,7 +164,9 @@
       wasm_exports.tdkpin_browser_high_score_ack(revision);
       lastHighScoreRevision = revision;
       highScoreSubmissionWarningShown = false;
+      resetHighScoreRetry();
     } catch (error) {
+      scheduleHighScoreRetry(retryAfterMs);
       if (!highScoreSubmissionWarningShown) {
         console.warn("shared high-score submission failed; will retry", error);
         highScoreSubmissionWarningShown = true;