fix: attach form submit listener before paint to prevent reload flake

[Archmonger]

Description

test_form_no_page_reload was flaky in CI (e.g. run 35571256796, python-source 3.13), failing with:

assert render_count.current == 1
E   assert 2 == 1

Root cause

The Form component (from #58) wires its submit event listener inside React.useEffect, which runs after the browser paints. That leaves a window — widest on a slow/loaded CI machine — after the form is rendered but before the submit listener is attached.

When a submit fires during that window, the listener's preventDefault() never runs, so the browser performs its native GET form submission → a full page reload. ReactPy remounts the app and re-runs the route component, so the server-side Ref render counter reaches 2 (and the client-side router/scroll store is wiped). This is the same class of defect as the Link preventDefault race that was fixed for scroll restoration in #59.

Fix

Attach the submit listener in React.useLayoutEffect instead of React.useEffect. useLayoutEffect runs synchronously before the browser paints, so the preventDefault handler is always in place by the time the form is visible and interactive — eliminating the fall-through-to-native-submit window entirely.

React is preact/compat from @reactpy/client, which provides useLayoutEffect.

Verification

Before the fix, the form-no-reload test reproduced render_count == 2 under load. After the fix:

• 15/15 CI-style hatch test --headless runs
• 27/27 genuine runs under taskset -c 0,1 -n 6 oversubscription (the only 3 "failures" were unrelated bun install EEXIST harness races from concurrent xdist workers)
• Full suite 30/30 passed across 3 runs
• Prettier/ESLint and pyright clean

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

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