refactor: reusable stepped-flow chrome; migrate currency-creator

[bmc08gt]

Summary

Extracts the stepped-wizard chrome that was baked into currency-creator into a reusable, opt-in flow type in apps/flipcash/core, and migrates currency-creator onto it. No behavior change to the currency-creator flow — this is groundwork so upcoming stepped flows (e.g. tipping setup) can reuse the same chrome instead of hand-rolling a top bar.

The underlying com.getcode.navigation.flow framework already had 8 consumers but provided no chrome; currency-creator was the only one with a progress-bar wizard. That pattern is now shared.

What's new (in com.flipcash.app.core.ui.flow)

• SteppedFlowRoute<R> — a FlowRouteWithResult that declares progressSteps. Declaring your route as this is how a flow opts into the progress chrome.
• SteppedFlowScaffold — wraps FlowHost, renders the progress top bar, and owns the progress math (reads route.progressSteps against the current step) so it no longer lives in the ViewModel.
• FlowStepBar + FlowStepBarController / LocalFlowStepBar — the generalized progress top bar and its state holder (progress / onBack / onEndAction).
• FlowStepper + StepperItem — the vertical intro stepper, now driven by a caller-supplied List<StepperItem> instead of hardcoded content.
• flowProgressFor(step, progressSteps) — the step→fraction helper, unit-tested.

currency-creator migration

• AppRoute.Token.CurrencyCreator is now a SteppedFlowRoute and declares its 5 progress steps.
• CurrencyCreatorFlowScreen uses SteppedFlowScaffold; per-step title/end content extracted into currencyCreatorTitleContent / currencyCreatorEndContent helpers.
• InfoScreen renders the shared FlowStepper (with a currencyCreatorStepperItems builder).
• Removed progress / connectTopBar from CurrencyCreatorViewModel; deleted the bespoke CurrencyCreatorTopBar + CurrencyCreatorTopBarController.

Testing

• New FlowProgressTest (5 cases) covers the progress computation lifted out of the ViewModel.
• :apps:flipcash:core and :apps:flipcash:features:currency-creator unit tests pass.
• :apps:flipcash:app:assembleDebug builds successfully.

Behavior is preserved by construction: same animated progress bar, same back/close handling, same "Next" action on Bill Customization, same steps excluded from the bar.