feat(ui): replace user text with profile image in sidebar header

- Replace 'Logged in as: [user]' text with profile avatar
- Use applesauce ProfileModel to fetch user's profile picture
- Display profile image in 33px square (same size as IconButton)
- Show fallback user icon when no profile image available
- Style avatar with same border radius and styling as IconButton
- Add tooltip showing user display name on hover

src/index.css

@@ -110,13 +110,28 @@ body {
   margin-bottom: 1rem;
 }
 
-.sidebar-header-bar .user-info {
-  flex: 1;
-  margin: 0;
-  font-size: 0.85rem;
-  white-space: nowrap;
+.profile-avatar {
+  width: 33px;
+  height: 33px;
+  border-radius: 6px;
   overflow: hidden;
-  text-overflow: ellipsis;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: #2a2a2a;
+  border: 1px solid #444;
+  flex-shrink: 0;
+  color: #ddd;
+}
+
+.profile-avatar img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+}
+
+.profile-avatar svg {
+  font-size: 1rem;
 }
 
 .sidebar-header-bar .toggle-sidebar-btn {