feat: add form component and use_form_data hook

[Archmonger]

Description

Implements a Form component a la React Router's Form, addressing #7.

Form submissions are intercepted client-side, serialized via FormData into JSON, navigated via pushState (GET-method), and the submitted data is made available through the use_form_data() hook. Non-file fields only; file serialization is deferred.

Changes

JavaScript:

• Added Form component (src/js/src/components.ts) that intercepts submit events via class-based selectors (same pattern as Link), serializes FormData to Record<string, string[]>, navigates with query params, and fires onSubmitCallback to the server.
• Added FormProps / FormSubmitData types to src/js/src/types.ts.

Python:

• form(attributes, *children) in src/reactpy_router/components.py — mirrors the link() pattern. Renders a <form> with a unique CSS class and the JS Form listener as a sibling.
• use_form_data() hook in src/reactpy_router/hooks.py — returns the most recently submitted form data (dict[str, list[str]]).
• FormDataState context wired into the router in routers.py so form data is available to all routed components.
• Both form and use_form_data exported from __init__.py.

Tests (6 new):

• test_simple_form — basic form submission navigates to action URL
• test_form_no_page_reload — verifies no full page reload on submit
• test_form_action_url_with_query_string — action URL with pre-existing query params
• test_form_use_form_data — submitted data retrievable via use_form_data()
• test_form_with_multiple_values — multiple checkbox values serialized correctly
• test_form_class_name — className attribute pass-through

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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