fix(amount-entry): reset amount when the selected token changes

[bmc08gt]

What

Reset the entered/selected amount when the token changes, mirroring the existing behaviour for a region/currency change.

Previously, switching tokens re-denominated an amount entry but left the typed value in place, so an amount entered in one token was silently reinterpreted in another.

Changes

• AmountEntryDelegate — new optional tokenChanges: Flow<*> trigger. Its init now resets the keypad on either a preferred-rate change (region/currency, existing) or a token change, via merge(observePreferredRate(), tokenChanges.distinctUntilChanged().drop(1)). drop(1) skips the initial token emission so an in-flight prefill isn't wiped on construction.
• Give-cash (CashScreenViewModel) and chat amount entry (ChatViewModel) pass tokenChanges = tokenCoordinator.observeSelectedTokenMint() — both are driven by the global selected token.
• Tip user modal (TippingCoordinator) — new init observer on the selected token that clears the modal's selected TipAmount (selectAmount(null)), the non-keypad counterpart to the delegate reset.

Intentionally excluded

• Withdrawal and Swap — their token is pinned per-flow (e.g. WithdrawalStep.Amount(mint)), not the global selected token, so a global change must not reset their amount.
• Tip amount-entry keypad — no in-screen token picker; the ViewModel is recreated per open.

Testing

• :apps:flipcash:shared:amount-entry, :shared:tipping, :features:cash, :features:messenger compile.
• :shared:tipping and :features:cash unit tests pass.