[Feature]: Progressive-enhancement form actions + optimistic updates #140

Pre-flight checklist

• I searched existing issues and did not find a duplicate request.
• I explained the problem or use case, not just the desired API.

Problem or use case
bQuery forms submit via JS (onSubmit) only — there is no progressive-enhancement model where a form posts to a server action and works even before/without client JS, and no optimistic-update primitive for instant feedback. React 19 added function action/formAction props on <form>/<input>/<button> with useActionState, useFormStatus, and useOptimistic; SvelteKit ships server form actions. bQuery, despite having both a forms module and a server module, does not connect them this way, which is a concrete gap for resilient apps.

Proposed solution
Add a form-action model that binds a form to a server action (via the server module / file-based route actions in #149), submits as a normal POST when JS is unavailable, and progressively enhances to a fetch-based submit with pending state when JS is present. Add an optimistic-update primitive so the UI can reflect the expected result immediately and reconcile on response. Must compose with the existing validation pipeline and with sessions/CSRF from #132.

Possible API or UX shape

import { formAction, useFormStatus, optimistic } from "@bquery/bquery/forms";

// binds to a server action; falls back to native POST without JS
const submit = formAction("/todos", { method: "POST" });

const { pending } = useFormStatus(submit);
const list = optimistic(todos, (cur, draft) => [...cur, draft]); // instant feedback

Alternatives considered
Keeping JS-only submit and documenting manual fetch patterns — leaves bQuery without the resilience and ergonomics React 19/SvelteKit now treat as standard. Building optimistic updates only in store — possible, but form submission is where users expect it, so the primitive should be reachable from forms.

Relevant area
forms

Additional context
Depends on server session/CSRF (#132) and pairs with file-based route actions (#149). This is one of the headline React-19-parity features.