feat(compose): the signature follows the account until it is chosen

A From: change re-seeds the signature from the newly selected account, and
stops doing so the moment the user picks one from the switch. Re-seeding
unconditionally is the one behaviour that can silently discard a deliberate
choice made a moment earlier; this is the shape send_html already uses.

seededSignatureName() reads the combo rather than the context, which
records where the composer opened and does not follow a change to it.

Part of item 152.

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