feat(frontend): unified async loading and error display components

[Shakalito]

Create reusable UI patterns for async states and API error handling.

AC:

• Shared ErrorBanner / LoadingState components

• Forms disabled during submit consistently

• Human-readable API error messages (no [object Object])

• Applied to Login/Register, Alerts, Portfolio trade forms

Deps

• feat(ui): migrate app shell to Tailwind CSS + shadcn (dark theme) #57