send: stop the Max buttons from cancelling their own submit, show LNURL action errors

Choosing Max on the LNURL-pay page left the button spinning forever. Two
things combined:

The Max buttons set `loading` from their click handler. Svelte flushes
`disabled={loading}` to the DOM before the browser runs the button's
submit activation, and a disabled button submits nothing. Set loading
from the form's onsubmit instead, the way the Send button always did.

The /ln page never passed `form` into the Lnurlp and Lnurlw components,
so any action failure (pay, max or withdraw) rendered nothing and never
reset the spinner. Pass it through, and reset loading when a result
arrives.

Verified in headless Chrome against the dev stack: the bolt11 Max lands
on the fee view, and the LNURL Max surfaces the server's error and
re-enables the buttons.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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