test: add browser extension QA harness

[0xpantera]

Summary

• Add a lightweight Playwright Test harness for the experimental Deckard browser extension.
• Load the unpacked Manifest V3 extension in bundled Chromium through a persistent context.
• Verify service worker loading and EIP-1193 provider behavior against the local test dapp in dev/mock bridge mode.
• Document the extension QA workflow and add branch-hygiene preflight rules to agent instructions.

Validation

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

Notes

• The Playwright suite uses a deterministic mock address only. It does not use real seeds, private keys, production wallet state, or mainnet funds.
• No Cargo.toml or Cargo.lock changes.