Track encounter start/end timestamps; display on card

startEncounter sets startedAt (clears endedAt on restart). endEncounter sets
endedAt. snapshotOf includes both for undo/redo fidelity. expandUndo split
start/end cases to restore old timestamps.

Display: encounter card shows Started/Ended independently (endedAt shows
even without startedAt). Undo sets null (can't delete via merge), test snap
strips only startedAt/endedAt null for compare.

Reviewed by pi gpt-5.5: stale endedAt on restart fixed, snap null-strip
narrowed. Medium-low risk.

src/App.js

@@ -2258,6 +2258,11 @@ function EncounterManager({ campaignId, initialActiveEncounterId, campaignCharac
                     <p className="text-xs text-stone-300">
                       {encounter.createdAt && `${new Date(encounter.createdAt).toLocaleDateString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false })} · `}Participants: {encounter.participants?.length || 0}
                     </p>
+                    {(encounter.startedAt || encounter.endedAt) && (
+                      <p className="text-xs text-green-400/80 mt-0.5">
+                        {encounter.startedAt && `⚔ Started: ${new Date(encounter.startedAt).toLocaleString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false })}`}{encounter.startedAt && encounter.endedAt && ' · '}{encounter.endedAt && `Ended: ${new Date(encounter.endedAt).toLocaleString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false })}`}
+                      </p>
+                    )}
                     {isLive && (
                       <span className="text-xs text-green-400 font-semibold block mt-1">
                         LIVE ON PLAYER DISPLAY