fix(fab-button): type="submit" and type="reset" now work inside forms

[LucasArray]

Summary

Fixes #18550 — ion-fab-button with type="submit" does not submit its parent form.

Root cause: ion-fab-button uses Shadow DOM, so its internal <button> element is not part of the form-associated elements in the light DOM. Clicking it (or pressing Enter in a text field) never triggers the form's submit event.

Fix: Port the hidden-button pattern already used by ion-button:

• On render (when type !== 'button'), inject a hidden <button> into the associated form in the light DOM.
• On click, call ev.preventDefault() to stop the shadow button from acting, then .click() the hidden button — which is a proper form participant and triggers submission.
• Add a form prop (string | HTMLFormElement) so the button can target a form outside its DOM subtree (same API as ion-button).
• Keep the disabled watcher in sync with the hidden button so a disabled fab-button cannot submit.

Changes

• core/src/components/fab-button/fab-button.tsx — core fix
• core/src/components/fab-button/test/form/fab-button.e2e.ts — Playwright e2e tests (closest form, form by id, form by reference, Enter key, disabled, reset)
• core/src/components/fab-button/test/form/fab-button.spec.ts — unit test asserting the hidden button is not duplicated on re-renders
• core/src/components/fab-button/test/form/index.html — visual test page

Test plan

• npm run test.spec passes (unit test for hidden-button dedup)
• npm run test.e2e passes for fab-button/test/form/fab-button.e2e.ts
• Manual: ion-fab-button type="submit" inside a <form> submits on click
• Manual: pressing Enter in a form input triggers submit
• Manual: ion-fab-button type="reset" resets form fields
• Manual: disabled fab-button does not submit
• Manual: form="id" targets an external form

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