feat(layout) : add collapsible sidebar with icon-only compact mode

Introduces SidebarLink component, UI store with localStorage persistence,
and smooth CSS transitions between expanded (w-64) and compact (w-16) modes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/stores/ui.ts

@@ -0,0 +1,22 @@
+export const useUiStore = defineStore('ui', () => {
+    const sidebarCollapsed = ref(false)
+
+    if (import.meta.client) {
+        const saved = localStorage.getItem('ui-sidebar-collapsed')
+        if (saved !== null) {
+            sidebarCollapsed.value = saved === 'true'
+        }
+    }
+
+    watch(sidebarCollapsed, (val) => {
+        if (import.meta.client) {
+            localStorage.setItem('ui-sidebar-collapsed', String(val))
+        }
+    })
+
+    function toggleSidebar() {
+        sidebarCollapsed.value = !sidebarCollapsed.value
+    }
+
+    return { sidebarCollapsed, toggleSidebar }
+})