fix(frontend): reject invalid WalletConnect signing requests

[cursor]

Motivation

Recent WalletConnect BTC/Solana signing changes introduced concrete request lifecycle failures:

• Solana signing could return { success: false } without sending a WalletConnect JSON-RPC rejection when the wallet address was unavailable or when a sign-and-send transaction could not produce a signature. The user flow stopped, but the dApp request stayed pending indefinitely.
• BTC signPsbt could approve a request with an unchanged PSBT when signInputs was present but selected no usable input indices, reporting success without signing anything.

Changes

• Reject Solana WalletConnect requests before returning failure for a missing wallet address.
• Reject Solana WalletConnect requests when signing cannot produce a signature.
• Reject BTC PSBT requests when signInputs is provided but resolves to no input indices, before opening the signing progress flow.
• Added regression coverage for the Solana rejection paths and BTC empty-signInputs PSBT path.

Tests

• PATH="/home/ubuntu/.nvm/versions/node/v24.17.0/bin:$PATH" npm run test -- --run src/frontend/src/tests/sol/services/wallet-connect.services.spec.ts src/frontend/src/tests/btc/services/wallet-connect.services.spec.ts
• PATH="/home/ubuntu/.nvm/versions/node/v24.17.0/bin:$PATH" npm run format
• PATH="/home/ubuntu/.nvm/versions/node/v24.17.0/bin:$PATH" npm run lint -- --max-warnings 0
• PATH="/home/ubuntu/.nvm/versions/node/v24.17.0/bin:$PATH" npm run check
• PATH="/home/ubuntu/.nvm/versions/node/v24.17.0/bin:$PATH" npm run test
• PATH="/home/ubuntu/.nvm/versions/node/v24.17.0/bin:$PATH" npm run lint:lockfile