feat: editor improvements and collapsible sidebars

Add collapse/expand toggle for the AppShell navigation sidebar and the
editor properties panel (both persisted to localStorage). Bundles other
in-progress editor work including position anchors, outlet sizing, PBR
textures, window slope/frame depth, curtain metadata, and various 2D/3D
rendering tweaks.

apps/client/src/components/editor/properties-panel.module.css

@@ -8,6 +8,10 @@
 }
 
 .header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--space-2);
   padding: var(--space-3) var(--space-4);
   font-size: var(--font-size-sm);
   font-weight: var(--font-weight-semibold);
@@ -17,6 +21,40 @@
   text-transform: uppercase;
 }
 
+.collapseBtn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 24px;
+  height: 24px;
+  padding: 0;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  background: var(--color-bg);
+  color: var(--color-text-secondary);
+  cursor: pointer;
+  font-size: 12px;
+  line-height: 1;
+  transition: background-color var(--transition-fast), color var(--transition-fast);
+}
+
+.collapseBtn:hover {
+  background-color: var(--color-bg-hover);
+  color: var(--color-text-primary);
+}
+
+.panelCollapsed {
+  width: 32px;
+  min-width: 32px;
+  background-color: var(--color-bg-elevated);
+  border-left: 1px solid var(--color-border);
+  flex-shrink: 0;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  padding-top: var(--space-2);
+}
+
 .empty {
   padding: var(--space-6) var(--space-4);
   text-align: center;