Fix keystone flow

[dewabisma]

Summary

• Now display details for tx in sign screen
• Now cache keystone session, to avoid bad flow when user just want to reconfirm by pressing back button.

Intentionally didn't disable back button, because don't want to block the flow. User might be stuck if we disable back button.

Screenshots

• Sign screen

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Note

Medium Risk
Changes hardware-wallet signing payload reuse and navigation; incorrect session clearing could show a stale QR, though new sends explicitly reset the cache.

Overview
Improves the Keystone hardware send flow by caching the unsigned payload and UR QR parts for the current send, and by showing amount, recipient, and checkphrase on the sign screen.

A new Riverpod keystoneSignCacheProvider holds a single entry keyed by account, recipient, and amount (block height and nonce are excluded so minor chain drift does not force a new QR while the user navigates back within the same send). KeystoneSignScreen reuses the cache on _prepare when the key matches; otherwise it builds, stores, and displays as before. startNewSendSession() clears the cache when the user starts a fresh send from home (Send, multisig propose, pay intent) or from the shared-address “Send to this account” action so a new flow does not reuse an old QR.

When continuing to signature scan, block height is taken from the cached unsigned payload’s blockNumber instead of the value passed into the sign screen. Unit tests cover cache keys (address trimming) and notifier store/lookup/session invalidation.

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