Verification compared the signed artifact against the dApp's request object.
For every field the dApp omitted -- normally nonce, gas limit and all the fee
fields, since the popup filled them in -- the number the user actually read on
screen was verified by nothing, and only absolute ceilings stood behind it.
The transaction is now populated in the background before the approval window
opens, and that populated object is both what the popup displays and what the
signed artifact is verified against. Every consequential field becomes an
equality comparison; the ceilings remain as a backstop. Population failing
means no approval and no window, and the error goes to the requesting page --
earlier than before, where the same estimate failed after the password had been
typed.
The account is pinned too: `from` is compared against the address named at
approval time rather than whichever address is active at signing, so switching
accounts mid-flow refuses instead of signing from an account the approval did
not name. The message-signing path had the same defect and gets the same fix.
Nonce selection moves earlier as a consequence; the concurrent-approval case
that follows from it is tracked at #271.
