[Amount] Improve formatting capabilities and fix edge cases

Summary

Improvements to the Amount component to enhance formatting capabilities, fix edge cases, and follow component conventions.

Improvements

2. Add notation prop

No way to get compact formatting like $1.2M or $13K. Intl.NumberFormat supports notation: 'compact' natively. This is useful for dashboards and summary views.

3. Add narrowSymbol to currencyDisplay

Intl.NumberFormat supports 'narrowSymbol' (e.g., $ instead of US$ in non-US locales), but the currencyDisplay prop only allows 'symbol' | 'code' | 'name'.

4. Add signDisplay prop

Intl.NumberFormat supports signDisplay ('auto', 'always', 'exceptZero', 'never') for controlling +/- sign display — useful for showing gains/losses.

5. Memoize formatter instances

getCurrencyDecimals() and isValidCurrency() each create a new Intl.NumberFormat instance on every render. These should be memoized or combined into a single formatter creation.

6. Fix string slicing edge case for short strings

value.slice(0, -decimals) + '.' + value.slice(-decimals) breaks for strings shorter than the decimal count (e.g., value="5" with USD → ".5" instead of "0.05").

Moved to Cross-Cutting Issues

The following items have been moved to cross-cutting issues that address them across all components:

• Forward HTML attributes (#674)
• TypeScript strictness (#675)

Start by locating the Amount component and its Intl.NumberFormat usage, then inspect the existing currencyDisplay props and formatting paths. The work is done when the requested notation, narrowSymbol, and signDisplay options are supported, formatter creation is memoized or combined, and short values format correctly.