Reject malformed custom slippage input - #104

Summary

• replace partial parseFloat handling with a strict decimal parser for custom slippage input
• reject malformed values such as 1abc, 1,5, 0.5%, exponent notation, zero, and values above 50 without calling onChange
• show an invalid state for rejected custom values while preserving normal decimal entry with inputMode="decimal"
• add focused component coverage for invalid and valid custom slippage values

Why

parseFloat accepts partial numeric prefixes, so malformed values can silently apply a different slippage than the text the user entered. This keeps displayed input and applied slippage aligned.

Closes #57.

Validation

• npm run test -- __tests__/components/SlippageSelector.test.tsx
• npm test -- --runInBand (26 tests passed)
• npm run lint -- components/SlippageSelector.tsx __tests__/components/SlippageSelector.test.tsx
• npm run typecheck
• git diff --check