docs: update READMEs for PSM SDK migration

[WiktorStarczewski]

Summary

The READMEs were outdated after #24 migrated the frontend from the coordinator REST API to the PSM SDK. This PR rewrites both to reflect the current architecture.

Root README.md

• Added architecture diagram showing PSM SDK flow (frontend → PSM + Miden RPC, coordinator server optional)
• Added wallet support table (Local keys, Para/ECDSA, Miden Wallet)
• Added complete environment variable reference for both frontend and server, including new PSM/Para vars
• Added local development section with prerequisites, dev tools, and .env.local example
• Added common Make targets reference

bin/coordinator-frontend/README.md

• Removed references to coordinator REST API as a requirement
• Added PSM SDK and WASM client to the description
• Added full env var table (NEXT_PUBLIC_PSM_ENDPOINT, NEXT_PUBLIC_PARA_API_KEY, etc.)
• Updated project structure to match current codebase (contexts/, hooks/, lib/, wallets/)
• Added key files section pointing to important source files
• Added troubleshooting section (WASM loading, Para wallet, stale state)