test(harness): bind the static test server to an ephemeral port

The session-scoped autouse fixture bound 127.0.0.1:7011 and raised when the
port was taken. Because it is autouse, that raise errored every collected
test rather than the browser ones: a second worktree running its own suite
produced 10,612 errors, none of them about the code under test. 7011 is also
the application's own default port, so the suite could not run while a local
instance was up.

Bind port 0 instead and publish the resulting origin as
ODYSSEUS_TEST_STATIC_ORIGIN. The browser tests shell out to node, which
inherits the environment, so the snippets read process.env rather than
hardcoding a port. ODYSSEUS_TEST_STATIC_PORT still pins one when something
outside pytest has to reach the server; that is the only path that can now
fail to bind, and it fails with a message that says so.

Two concurrent full runs from one checkout now both pass. Only the docx export
snippet is an rf-string, so it is the only one whose JS braces needed doubling.

tests/test_document_rich_table_headers.py

@@ -39,7 +39,7 @@ def test_mobile_header_row_and_column_toggle_independently_with_undo():
       import { chromium } from 'playwright';
       const browser = await chromium.launch({ headless: true });
       const page = await browser.newPage({ viewport: { width: 390, height: 844 } });
-      await page.goto('http://127.0.0.1:7011/static/js/documentStats.js');
+      await page.goto(`${process.env.ODYSSEUS_TEST_STATIC_ORIGIN}/static/js/documentStats.js`);
       await page.setContent('<link rel="stylesheet" href="/static/style.css?v=20260831richtexttools91"><div id="toast"></div><div id="chat-container"></div><div id="sidebar"></div>');
       await page.evaluate(async () => {
         const mod = await import('/static/js/document.js?v=20260831richtexttools91&table-header-test=1');