Surface custom plugin updates in the WebUI

[3clyp50]

Summary

• run daily custom-plugin update checks in the background without delaying user messages
• preserve the last good status across remote failures and retry only failed plugins hourly
• show a persistent toast that opens the Plugins modal in the Custom tab
• show Update actions for custom plugins and reuse the existing installer confirmation and update flow
• clear advisory update state after a successful plugin update without turning cleanup failures into update failures

Verification

• conda run -n a0 python -m pytest tests/test_plugin_update_check.py tests/test_plugin_git_update.py tests/test_plugin_activation_ui.py tests/test_download_toast_regressions.py tests/test_defer_lifecycle.py -q (29 passed)
• JavaScript syntax checks for both changed stores
• Python compilation and git diff --check
• live Docker runtime at http://localhost:32081: source/runtime hashes matched, HTTP 200, background update hook returned in 1.1 ms, toast opened the existing Custom plugins modal without duplication, and the browser console stayed clean

[3clyp50]

Closing because this change was intended for the origin repository instead: 3clyp50#22