M2: refactor DisplayView to storage adapter (red test first)

DisplayView missed in original M2 refactor — raw onSnapshot(doc(db,path))
survived. In ws/memory mode db is a stub sentinel, so raw SDK calls crash
('Expected first argument to collection() to be a CollectionReference...').
Reported by human testing player display after start combat.

TDD:
1. RED: DisplayView.characterization.test.js asserts adapter.subscribeDoc
   called for campaign + encounter + activeDisplay paths. Adapter recorder
   (getAdapterCalls/resetAdapterCalls in firebase.js) instruments subscribe
   calls — catches raw-SDK bypass that firebase mock alone cannot (mock db
   satisfies raw onSnapshot, hiding the bug).
2. Fix: 2 raw onSnapshot sites in DisplayView -> storage.subscribeDoc.
3. GREEN: 2 new tests pass, 116 total green.

Audit confirmed DisplayView was the ONLY remaining raw SDK site in App.js.

src/App.js

@@ -2453,35 +2453,23 @@ function DisplayView() {
         setIsLoadingEncounter(true);
         setEncounterError(null);
 
-        const campaignDocRef = doc(db, getPath.campaign(activeCampaignId));
-        unsubscribeCampaign = onSnapshot(
-          campaignDocRef,
-          (campSnap) => {
-            if (campSnap.exists()) {
-              setCampaignBackgroundUrl(campSnap.data().playerDisplayBackgroundUrl || '');
-            } else {
-              setCampaignBackgroundUrl('');
-            }
-          },
-          (err) => console.error("Error fetching campaign background:", err)
+        unsubscribeCampaign = storage.subscribeDoc(
+          getPath.campaign(activeCampaignId),
+          (camp) => {
+            setCampaignBackgroundUrl((camp && camp.playerDisplayBackgroundUrl) || '');
+          }
         );
 
-        const encounterPath = getPath.encounter(activeCampaignId, activeEncounterId);
-        unsubscribeEncounter = onSnapshot(
-          doc(db, encounterPath),
-          (encDocSnap) => {
-            if (encDocSnap.exists()) {
-              setActiveEncounterData({ id: encDocSnap.id, ...encDocSnap.data() });
+        unsubscribeEncounter = storage.subscribeDoc(
+          getPath.encounter(activeCampaignId, activeEncounterId),
+          (enc) => {
+            if (enc) {
+              setActiveEncounterData({ id: activeEncounterId, ...enc });
             } else {
               setActiveEncounterData(null);
               setEncounterError("Active encounter data not found.");
             }
             setIsLoadingEncounter(false);
-          },
-          (err) => {
-            console.error("Error fetching active encounter details:", err);
-            setEncounterError("Error loading active encounter data.");
-            setIsLoadingEncounter(false);
           }
         );
       } else {