chore: convert React frontend to TypeScript

[gaidheal1]

Overview

Migrate the React frontend from JavaScript to TypeScript to improve type safety, IDE support, and long-term maintainability.

Motivation

• Catch type errors at compile time rather than runtime
• Better IDE autocomplete and refactoring support
• Clearer component prop contracts

Scope

• Add TypeScript dependencies (typescript, @types/react, @types/react-dom, etc.)
• Update build config (Vite/Webpack/CRA depending on setup)
• Rename .js/.jsx files to .ts/.tsx
• Add type annotations incrementally, starting with shared utilities and API types
• Configure tsconfig.json (recommend strict: false initially, tighten later)

Acceptance criteria

• Project compiles with no TypeScript errors
• Existing tests continue to pass
• CI pipeline updated if needed

Notes

Can be done incrementally — TypeScript supports mixed JS/TS codebases during migration. Recommend starting with new files and migrating existing ones opportunistically.