[web] Autofill all AutofillGroup fields in semantics mode (#187108)

**Fixes #180652**

  **Problem**

With a screen reader on, picking a saved login only filled the field you
tapped. The other fields stayed empty.
  
Browsers autofill a login by putting all its fields in one hidden
`<form>`. But in screen-reader mode each field has to live in its own
spot for accessibility, so it never went into that form. The browser saw
a half-built login and filled only one field.

  **Fix**

Still build the hidden form, but instead of moving the field into it,
just link them with the HTML `form="<id>"` attribute. The field stays
where the screen reader needs it, and the browser now sees a complete
login and fills every field.

  **Demo**
  
  Semantics ON:
  - Before: https://flutter-demo-30-before.web.app — only email fills
  - After: https://flutter-demo-30-after.web.app — both fields fill

  Semantics OFF (regression check, both behave the same):
  - Before: https://flutter-demo-31-before.web.app
  - After: https://flutter-demo-31-after.web.app

**Known limitation**

Chrome on iOS still fills only the username, not the password. That is a
pre-existing WebKit limitation unrelated to this change (it also happens
without a screen reader) and is tracked separately in #185327.

---------

Co-authored-by: Harry Terkelsen <1961493+harryterkelsen@users.noreply.github.com>