Add end-to-end type safety with contract types

[big14way]

Generate TypeScript types from the Soroban contract to ensure type safety across the frontend.

Acceptance Criteria:

• Set up type generation from contract WASM or ABI
• Replace manually typed interfaces with generated types
• Add type generation to build script
• Verify all contract interactions use generated types

Files: src/lib/sorosave.ts, package.json, new file src/types/contract.ts