Fix Docker plugin installation failures

[BlizzHacker]

Fixes #15

What changed

• install git in the final Docker image because ROM Hub installs catalogue entries from pinned repository refs
• make the Docker smoke test install the bundled Archive.org plugin, covering both the runtime dependency and the catalogue tag
• preserve and display /api/v1/hub/plugin errors instead of immediately refreshing them away
• handle non-JSON and network failures with a useful fallback message

The corresponding Archive.org v0.4.0 source is published at the tag expected by the bundled catalogue. Its review PR is BlizzHacker/rom-hub-archive-org#2.

Verification

• python -m pytest -q tests/test_docker_install.py tests/test_ui.py tests/test_plugin_trust.py tests/test_plugin_sandbox.py — 36 passed
• pinned ROM Hub install probe — archive-org 0.4.0 enabled [census,importer,metadata,search,stream]
• full suite — 1,976 passed, 1 skipped, 1 deselected; only two pre-existing shell syntax probes could not run because this Windows host resolves the Microsoft Store WSL placeholder as bash.exe