fix: detect identity reset by listening to cross-signing key changes

[eleboucher]

Description

When the server/IdP resets identity, the Matrix SDK fires CryptoEvent.KeysChanged, but the client only re-evaluated verification status on CryptoEvent.DevicesUpdated. So after a reset the client kept showing "Verified" and never surfaced the unverified state.
Adds useCrossSigningKeysChange (mirroring useDeviceListChange) and wires it into both verification hooks so status and unverified-device count re-evaluate on key changes. The existing unverified banner and Devices page then reflect the reset.

Fixes #1255

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).