fix: sync packet hash into URL after trace (#1523)

Closes #1522

## Summary

- Call `history.replaceState` in `doTrace()` after the hash is
validated, so the URL becomes `#/tools/trace/<hash>` and can be shared
directly.

## Change

`public/traces.js` — one line added:
```js
history.replaceState(null, '', `#/tools/trace/${encodeURIComponent(hash)}`);
```

The read path (`init()` picks up the hash from the URL on load) already
existed — only the write path was missing.

public/traces.js

@@ -51,6 +51,7 @@
     const hash = input.value.trim();
     if (!hash) return;
     currentHash = hash;
+    history.replaceState(null, '', `#/tools/trace/${encodeURIComponent(hash)}`);
 
     const results = document.getElementById('traceResults');
     results.innerHTML = '<div class="text-center text-muted" style="padding:40px">Tracing…</div>';

test-issue-1522-trace-url-sync-e2e.js

@@ -0,0 +1,26 @@
+// E2E test for issue #1522 — trace hash written into URL after doTrace().
+// Run: npx playwright test test-issue-1522-trace-url-sync-e2e.js
+// Requires: running server on BASE_URL (default http://localhost:3000).
+'use strict';
+
+const { test, expect } = require('@playwright/test');
+const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
+
+test.describe('Trace URL sync — issue #1522', () => {
+  test('hash written into URL after clicking Trace', async ({ page }) => {
+    await page.goto(`${BASE_URL}/#/tools/trace/`);
+    await page.waitForSelector('#traceHashInput', { timeout: 8000 });
+    await page.fill('#traceHashInput', 'deadbeef');
+    await page.click('#traceBtn');
+    await expect(page).toHaveURL(/#\/tools\/trace\/deadbeef$/);
+  });
+
+  test('deep-link pre-fills input and fires trace on load', async ({ page }) => {
+    await page.goto(`${BASE_URL}/#/tools/trace/cafebabe`);
+    await page.waitForSelector('#traceHashInput', { timeout: 8000 });
+    const value = await page.inputValue('#traceHashInput');
+    expect(value).toBe('cafebabe');
+    // URL must remain unchanged (no extra replaceState pollution)
+    await expect(page).toHaveURL(/#\/tools\/trace\/cafebabe$/);
+  });
+});