Initial data model migration

less/windows.less

@@ -0,0 +1,70 @@
+// Window and sheet styles
+
+.window-app {
+  .window-header {
+    font-family: @font-family-base;
+
+    .window-title {
+      font-size: @font-size-medium;
+      font-weight: normal;
+      color: @color-text-light;
+    }
+  }
+
+  .window-content {
+    background: @color-background;
+    padding: @spacing-medium;
+  }
+}
+
+// Sheet sections
+.sheet-header {
+  .flex-row();
+  padding: @spacing-medium;
+  border-bottom: 2px solid @color-primary;
+  margin-bottom: @spacing-medium;
+
+  img.profile-img {
+    flex: 0 0 100px;
+    height: 100px;
+    object-fit: cover;
+    border: 2px solid @color-border;
+    border-radius: @input-border-radius;
+    cursor: pointer;
+
+    &:hover {
+      border-color: @color-primary;
+    }
+  }
+
+  .header-fields {
+    flex: 1;
+    padding-left: @spacing-medium;
+  }
+}
+
+.sheet-body {
+  overflow-y: auto;
+
+  section {
+    margin-bottom: @spacing-large;
+  }
+}
+
+// Edit/Play mode toggle
+.sheet-mode-toggle {
+  position: absolute;
+  top: @spacing-base;
+  right: 80px;
+  z-index: 10;
+
+  button {
+    padding: @spacing-small @spacing-base;
+    font-size: @font-size-small;
+
+    &.active {
+      background: @color-primary;
+      color: @color-text-light;
+    }
+  }
+}