Add provider events to browser bridge

[0xpantera]

Summary

• add minimal EIP-1193 provider event support and isConnected() to the injected provider
• verify accountsChanged/removeListener and disconnect behavior through mock and real-daemon extension QA
• clarify Kohaku is a provider/backend abstraction, not Deckard's browser-facing dapp provider

Verification

• npm run qa:extension
• npm run qa:extension:real
• cargo fmt --all --check
• just check
• cargo test --workspace