fix(moderation): neutral phrasing for paginated member events without prev_content

Fixes #82

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room-settings/RoomActivityLog.tsx

@@ -63,8 +63,26 @@ function describeEvent(mx: ReturnType<typeof useMatrixClient>, ev: MatrixEvent):
       const prevMembership = prevContent.membership as string | undefined;
       const reason = content.reason as string | undefined;
       const targetName = getDisplayName(mx, stateKey);
+      // getPrevContent() falls back to {} when unsigned.prev_content is missing,
+      // which is common for state events fetched via back-pagination (homeservers
+      // don't always include it in /messages). That makes prevMembership
+      // indistinguishable from "no prior membership" (fresh join). Check the raw
+      // unsigned field so we only apply the join/leave-transition assumptions
+      // below to events where the SDK actually gave us prior state.
+      const hasPrevContent = ev.getUnsigned().prev_content !== undefined;
 
       if (membership === 'join') {
+        if (!hasPrevContent) {
+          return {
+            text: (
+              <>
+                <strong>{targetName}</strong>&apos;s membership changed to <strong>joined</strong>
+              </>
+            ),
+            iconSrc: Icons.User,
+            filter: 'members',
+          };
+        }
         if (
           prevMembership === 'invite' ||
           prevMembership === 'knock' ||
@@ -94,6 +112,17 @@ function describeEvent(mx: ReturnType<typeof useMatrixClient>, ev: MatrixEvent):
       }
 
       if (membership === 'leave') {
+        if (!hasPrevContent) {
+          return {
+            text: (
+              <>
+                <strong>{targetName}</strong>&apos;s membership changed to <strong>left</strong>
+              </>
+            ),
+            iconSrc: Icons.User,
+            filter: 'members',
+          };
+        }
         if (prevMembership === 'ban') {
           return {
             text: (