fix: don't reset <select> elements to the first option after a remote form submission

[RazinShafayet2007]

Fixes #16093

After a successful remote-form submission, the framework called native form.reset() to clear the form. For <select> elements, native reset restores whichever <option> has the selected attribute, or falls back to the first option if none does — and since most <select> markup doesn't declare an explicit default, every successful submission caused the select to snap back to its first option, discarding what the user actually picked.

This PR replaces that reset call with read_default_values(), which resolves each field to its correct post-submit state explicitly rather than delegating to the browser. Text inputs, checkboxes, radios, and textareas keep the same behavior as before (defaultValue/defaultChecked). For <select>, a genuine, non-disabled declared default is restored just like defaultValue on a text input, while a select with no usable default — none declared, or only a disabled placeholder like <option selected disabled> — preserves the submitted value instead, captured at submit time to avoid a race condition with the DOM. The same logic applies to multi-select. Tests cover all four resulting behaviors, including multi-select restoring every declared default.

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[vercel]

In read_default_values, the captured submitted value is looked up with the raw DOM element name, but submitted_data keys were normalized by convert_formdata (stripped n:/b: prefix and [] suffix, nested paths), so the lookup always misses for prefixed/suffixed/nested select names and falls back to the stale DOM.

[ottomated]

This is pretty complicated for what should just be defaultValue on <select> elements. .as('select', 'value') can't set selected on the child option because it's just spread onto the parent <select>. Maybe requires a change in svelte to add defaultValue to <select>s?

(also, this PR won't work for when form.reset() is called outside of submission and the tests should be consolidated)

[RazinShafayet2007]

@ottomated You were right, there was a real gap in form.reset() thing. To fix that, added e.preventDefault() in handle_reset and pointed it at the same read_default_values() helper the submit path already uses, just with undefined instead of the submitted data, since there's nothing to fall back to on a manual reset. So hitting a reset button (or calling .reset() from anywhere) won't trigger native reset's broken select behavior anymore either. Threw in a test that clicks an actual <button type="reset"> so this doesn't quietly break later.

I agree it's not the prettiest but the complexity comes from working around the DOM's limits, <select> has no defaultValue. The defaultSelected && !disabled heuristic mirrors what defaultValue/defaultChecked do for other elements, just expressed through child options since that's the only API available. Although I'd grant that a Svelte-level defaultValue on <select> would make this cleaner.