Read the instructions panel's visibility on mount from
chrome.storage.local['goon_instructions_visible'] (absent => shown, the
first-time default) and persist the new value on every user toggle, so a
'hide instructions' choice survives popup reloads and service-worker
termination and is respected across the app via the shared storage key.
Adds unit tests: stored-hidden honored on mount, persistence on both toggle
directions, and first-time (absent key) defaulting to shown.
