feat(viewer): persist hide/show instructions state in chrome.storage.local

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.

src/viewer/index.js

@@ -6,6 +6,11 @@
 
 const STORAGE_KEY = 'goon_videos';
 const TAGGED_KEY = 'goon_tagged';
+// Durable "hide instructions" toggle. Absent key === SHOWN (true), matching the
+// first-time "expanded on mount" behavior. chrome.storage.local is extension-wide,
+// so the viewer window and every popup instance read/write the same value — the
+// preference is respected application-wide and survives reloads / SW termination.
+const INSTRUCTIONS_VISIBLE_KEY = 'goon_instructions_visible';
 
 // Set to true to enable verbose console output in the popup.
 const DEBUG = true;  // TEMP: enabled for debugging — revert to false when done
@@ -48,9 +53,22 @@ function setInstructionsExpanded(expanded) {
 }
 
 if (instructionsToggle && instructionsBody) {
-  setInstructionsExpanded(true);
+  // On mount, read the stored visibility. An absent key means SHOWN (true), so a
+  // first-time user still sees the steps. The viewer window reads the same
+  // extension-wide key, so the choice is respected across the whole app.
+  chrome.storage.local.get(INSTRUCTIONS_VISIBLE_KEY).then(stored => {
+    const visible = stored[INSTRUCTIONS_VISIBLE_KEY];
+    setInstructionsExpanded(visible === undefined ? true : visible);
+  }).catch(() => {
+    // Any read failure falls back to shown.
+    setInstructionsExpanded(true);
+  });
+  // On every user toggle, apply in-DOM and persist. Both directions write, so
+  // the state survives reloads and SW termination until the user flips it again.
   instructionsToggle.addEventListener('click', () => {
-    setInstructionsExpanded(instructionsBody.hidden);
+    const next = instructionsBody.hidden;
+    setInstructionsExpanded(next);
+    chrome.storage.local.set({ [INSTRUCTIONS_VISIBLE_KEY]: next });
   });
 }