fix: scope hide-balances to the home screen

[n13]

Problem

isBalanceHiddenProvider was read inside the amount formatters — balanceDisplayProvider and txAmountDisplayProvider in currency_display_provider.dart. Every screen that formats an amount goes through that provider, so the eye toggle on home masked amounts that have nothing to do with the account balance:

• multisig open proposals (ProposalListTile) and decoded calls in the approve/execute/cancel confirm sheets
• POS charge + QR screens
• send input amount, review send
• transaction detail sheet

Change

The formatter no longer has any notion of hidden balances. The home screen owns the flag and passes it explicitly to the components it contains:

• AmountDisplayWithConversion.isHidden → masks the main balance
• ActivitySection.isHidden / MultisigActivitySection.isHidden → masks the amounts in the home transaction list

Everything else formats amounts normally, always. Open multisig proposals are not masked even on a multisig home — they are pending decisions to review, not balance disclosure.

The toggle itself still persists via SettingsService; only the home screen reads it now.

DRY

• txItemAmountText(data, format, {isHidden}) in tx_item.dart replaces three copies of the hideAmount ? '—' : format(...).primaryAmount ternary (home, multisig, activity screen)
• the formatter's inline function type is now a TxAmountFormatter typedef
• customHiddenText is gone — its only caller passed the same '-----' the default already used; the placeholder now lives once, next to the component that renders it
• _toFiatDisplayState lost two parameters and its post-hoc mutation, so it just builds and returns the state

Verification

• dart analyze lib test in mobile-app: no issues
• flutter test in mobile-app: 291 tests pass
• melos run format clean

(melos run analyze can't complete locally — it dies in flutter pub get on a firebase_messaging plugin rsync error unrelated to this change.)

[dewabisma]

Make sense to only hide in home. Just wondering if user ever want show activity history screen with balance hidden? Maybe not.

LGTM!