Feat: Add collapsible shell

Frontend/src/components/shell/WorkspaceSidebar/WorkspaceSidebar.module.scss

@@ -1,6 +1,5 @@
 .sidebar {
   --sidebar-nav-item-min-height: var(--control-size-lg);
-  --sidebar-dock-clearance: 8rem;
   position: relative;
   min-width: 0;
   min-height: 0;
@@ -15,13 +14,60 @@
 
 .header {
   display: grid;
+  gap: var(--space-3);
+}
+
+.headerActions {
+  display: grid;
+  grid-template-columns: repeat(4, minmax(0, 1fr));
   gap: var(--space-2);
+  justify-items: stretch;
+}
+
+.headerControls {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr);
+  align-items: start;
 }
 
 .headerDrawerOpen {
   z-index: 4;
 }
 
+.headerActionButton {
+  width: 100%;
+  min-height: var(--control-size-md);
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  border: 1px solid color-mix(in srgb, var(--color-border-strong) 44%, transparent);
+  border-radius: var(--radius-pill);
+  background: color-mix(in srgb, var(--color-surface) 94%, transparent);
+  color: var(--color-text-muted);
+  box-shadow: var(--shadow-soft);
+  transition:
+    background 160ms var(--easing-standard),
+    color 160ms var(--easing-standard),
+    border-color 160ms var(--easing-standard),
+    transform 180ms var(--easing-standard);
+}
+
+.headerActionButton:hover,
+.headerActionButton:focus-visible {
+  background: var(--color-surface-hover);
+  border-color: var(--color-border);
+  color: var(--color-text);
+}
+
+.headerActionButton:hover {
+  transform: translateY(-1px);
+}
+
+.headerCollapseButton {
+  background: color-mix(in srgb, var(--color-accent-soft) 58%, transparent);
+  color: var(--color-accent-strong);
+}
+
 .section {
   display: grid;
   grid-template-rows: auto minmax(0, 1fr);
@@ -45,7 +91,7 @@
   overflow-y: auto;
   overscroll-behavior: contain;
   padding-right: var(--space-1);
-  padding-bottom: calc(var(--space-4) + var(--sidebar-dock-clearance));
+  padding-bottom: calc(var(--space-4) + var(--sidebar-dock-clearance, 8rem));
   margin-right: calc(var(--space-1) * -1);
 }
 
@@ -97,6 +143,50 @@
   color: var(--color-text-muted);
 }
 
+.sidebarCollapsed {
+  gap: var(--space-3);
+  padding: var(--space-3) var(--space-2);
+}
+
+.sidebarCollapsed .headerActions {
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+}
+
+.sidebarCollapsed .headerControls {
+  justify-items: center;
+}
+
+.sidebarCollapsed .header {
+  justify-items: center;
+}
+
+.sidebarCollapsed .navScroller {
+  padding-right: 0;
+  margin-right: 0;
+}
+
+.sidebarCollapsed .navItem {
+  grid-template-columns: auto;
+  justify-content: center;
+  gap: 0;
+  min-height: calc(var(--control-size-lg) - var(--space-1));
+  padding: var(--space-2);
+  border-radius: var(--radius-md);
+}
+
+.sidebarCollapsed .label,
+.sidebarCollapsed .itemMeta {
+  display: none;
+}
+
+.sidebarCollapsed .icon {
+  opacity: 1;
+}
+
+.sidebarCollapsed .section {
+  gap: var(--space-3);
+}
+
 @include respond-down(mobile) {
   .sidebar {
     display: none;