[plugin] Release v0.4.0

Package plugin v0.4.0.

Bumps plugin/manifest.json to 0.4.0 and adds the signed release zip under plugin/zip/v0.4.0/ (joinstr.zip + joinstr.zip.asc, signed with key 2D230520212A5901).

This is the first release carrying the headless daemon/cli support and aut-ct sybil resistance from !30 (merged) and concurrent per-pool coinjoin. The zip now packages the refactored module set (joinstr.py, cmdline.py, autct.py) alongside qt.py/lrs.py.