Add token picker so users can stream any asset, not just XLM

[Omoboi-dev]

The contract's create_stream already accepts any token: Address (any SAC-compliant asset), but the frontend hardcodes XLM in labels, amount formatting, and the create form.

What to do

• Add a token selector to src/components/CreateStreamForm.tsx — start with XLM + USDC on testnet, defined in src/config.ts (address + symbol + decimals).
• Thread the chosen token address through stream creation.
• Replace hardcoded "XLM" labels in src/components/StreamCard.tsx with the stream's actual token symbol, and make formatTokenAmount use that token's decimals.

Acceptance criteria

• User can pick a token when creating a stream.
• Cards display the correct token symbol and decimals.
• pnpm exec tsc --noEmit passes.

Pairs with contract issue #10 (multi-token support).