feat(wallet): cycle balance display between sats, fiat, and hidden

[dmnyc]

Summary

Android port of iOS feat/wallet-balance-toggle. Tapping the wallet
dashboard balance now cycles through three states instead of the
prior plain hide/show toggle:

1. SATS — current default. "21,000 sats" or app-wide fiat when
   the global `FiatPreferences.isFiatMode` flag is on.
2. FIAT — wallet-screen-scoped fiat. Renders the balance + each
   tx row's amount/fee in the user's currently-selected fiat currency
   without flipping the app-wide flag, so feed sat counts elsewhere
   keep showing sats.
3. HIDDEN — masks the dashboard balance AND every per-row amount
   • fee in the transaction history view, for screenshots /
     shoulder-surfing.

Storage

New per-pubkey key `walletBalanceDisplay_` (matches iOS) in
the existing `wisp_settings` SharedPreferences. The legacy global
`balance_hidden` Bool is read once per pubkey when no per-pubkey
entry exists (true → HIDDEN, false → SATS) and then a migrated value
is written. Legacy key left in place — older builds rolled back keep
their preference.

`WalletBalanceDisplayMode` enum exposes `next()` for the cycle and
`read` / `write` helpers. Both `WalletHomeContent` and
`TransactionRow` read the same key so dashboard + history stay in
lockstep.

Test plan

• Wallet dashboard: tap the balance once → it renders as fiat
  ("$12.34" using the user's selected fiat currency). Tap
  again → "* * * * *" hidden. Tap again → back to sats.
• Cycle persists across app restarts: leave the wallet in FIAT,
  kill + relaunch, dashboard opens in FIAT.
• In HIDDEN, navigate to Transactions → every row's amount + fee
  is masked ("+* * " / "- * *"; fee text masked). Tap the
  dashboard balance to leave HIDDEN → tx history numbers reappear
  on next open.
• In FIAT, transactions list amounts + fees render in the user's
  fiat currency.
• Wallet-screen FIAT state does NOT flip the app-wide
  `isFiatMode` flag: Settings → Interface → Fiat toggle stays
  whatever it was; feed action-bar zap counts, timestamps, etc.
  still respect the global setting.
• User who previously had `balance_hidden = true` opens this
  build for the first time → dashboard opens in HIDDEN. User who
  had it `false` → opens in SATS. Same value reflected after
  toggling and relaunching.
• User with no prior preference → defaults to SATS.
• Account switch: signing out and into a different pubkey starts
  that pubkey at its own default (SATS, since the per-pubkey key
  isn't migrated for the new account).
• FIAT mode when the exchange-rate cache hasn't loaded yet: falls
  back to the sats display so the dashboard doesn't show a blank.