feat(sidebar): add collapsible sections with localStorage persistence

Sidebar sections can now be collapsed/expanded by clicking the header.
State is persisted to localStorage so sections remain open/closed
across page navigation and browser sessions.

Changes:
- init-alpine.js: Added openSections state, toggleSection(),
  expandSectionForCurrentPage(), and localStorage helpers
- sidebar.html: Refactored with Jinja2 macros for DRY code,
  added collapsible sections with CSS transitions and rotating
  chevron icons

Features:
- Click section header to toggle expand/collapse
- Chevron rotates 180 degrees when expanded
- Smooth CSS transitions (no extra Alpine plugins needed)
- State persists in localStorage (admin_sidebar_sections key)
- Default: Platform Administration open, others closed
- Dashboard and Settings always visible (not collapsible)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

static/admin/js/init-alpine.js

@@ -1,15 +1,15 @@
 /**
  * Alpine.js v3 global data initialization
- * Provides theme toggle, menu controls, and page state
+ * Provides theme toggle, menu controls, sidebar sections, and page state
  */
 function data() {
+  // ─────────────────────────────────────────────────────────────────
+  // Theme (dark mode) persistence
+  // ─────────────────────────────────────────────────────────────────
   function getThemeFromLocalStorage() {
-    // if user already changed the theme, use it
     if (window.localStorage.getItem('dark')) {
       return JSON.parse(window.localStorage.getItem('dark'))
     }
-
-    // else return their preferences
     return (
       !!window.matchMedia &&
       window.matchMedia('(prefers-color-scheme: dark)').matches
@@ -20,12 +20,74 @@ function data() {
     window.localStorage.setItem('dark', value)
   }
 
+  // ─────────────────────────────────────────────────────────────────
+  // Sidebar sections persistence
+  // ─────────────────────────────────────────────────────────────────
+  const SIDEBAR_STORAGE_KEY = 'admin_sidebar_sections';
+
+  // Default state: Platform Administration open, others closed
+  const defaultSections = {
+    platformAdmin: true,
+    contentMgmt: false,
+    devTools: false,
+    monitoring: false
+  };
+
+  function getSidebarSectionsFromStorage() {
+    try {
+      const stored = window.localStorage.getItem(SIDEBAR_STORAGE_KEY);
+      if (stored) {
+        return { ...defaultSections, ...JSON.parse(stored) };
+      }
+    } catch (e) {
+      console.warn('Failed to parse sidebar sections from localStorage:', e);
+    }
+    return { ...defaultSections };
+  }
+
+  function saveSidebarSectionsToStorage(sections) {
+    try {
+      window.localStorage.setItem(SIDEBAR_STORAGE_KEY, JSON.stringify(sections));
+    } catch (e) {
+      console.warn('Failed to save sidebar sections to localStorage:', e);
+    }
+  }
+
+  // Map pages to their parent sections
+  const pageSectionMap = {
+    // Platform Administration
+    companies: 'platformAdmin',
+    vendors: 'platformAdmin',
+    users: 'platformAdmin',
+    customers: 'platformAdmin',
+    marketplace: 'platformAdmin',
+    // Content Management
+    'platform-homepage': 'contentMgmt',
+    'content-pages': 'contentMgmt',
+    'vendor-theme': 'contentMgmt',
+    // Developer Tools
+    components: 'devTools',
+    icons: 'devTools',
+    testing: 'devTools',
+    'code-quality': 'devTools',
+    // Platform Monitoring
+    imports: 'monitoring',
+    logs: 'monitoring'
+  };
+
   return {
+    // ─────────────────────────────────────────────────────────────────
+    // Theme
+    // ─────────────────────────────────────────────────────────────────
     dark: getThemeFromLocalStorage(),
     toggleTheme() {
       this.dark = !this.dark
       setThemeToLocalStorage(this.dark)
     },
+
+    // ─────────────────────────────────────────────────────────────────
+    // Mobile side menu
+    // ─────────────────────────────────────────────────────────────────
     isSideMenuOpen: false,
     toggleSideMenu() {
       this.isSideMenuOpen = !this.isSideMenuOpen
@@ -33,6 +95,10 @@ function data() {
     closeSideMenu() {
       this.isSideMenuOpen = false
     },
+
+    // ─────────────────────────────────────────────────────────────────
+    // Notifications menu
+    // ─────────────────────────────────────────────────────────────────
     isNotificationsMenuOpen: false,
     toggleNotificationsMenu() {
       this.isNotificationsMenuOpen = !this.isNotificationsMenuOpen
@@ -40,6 +106,10 @@ function data() {
     closeNotificationsMenu() {
       this.isNotificationsMenuOpen = false
     },
+
+    // ─────────────────────────────────────────────────────────────────
+    // Profile menu
+    // ─────────────────────────────────────────────────────────────────
     isProfileMenuOpen: false,
     toggleProfileMenu() {
       this.isProfileMenuOpen = !this.isProfileMenuOpen
@@ -47,11 +117,37 @@ function data() {
     closeProfileMenu() {
       this.isProfileMenuOpen = false
     },
+
+    // ─────────────────────────────────────────────────────────────────
+    // Pages menu (legacy)
+    // ─────────────────────────────────────────────────────────────────
     isPagesMenuOpen: false,
     togglePagesMenu() {
       this.isPagesMenuOpen = !this.isPagesMenuOpen
     },
+
+    // ─────────────────────────────────────────────────────────────────
+    // Collapsible sidebar sections
+    // ─────────────────────────────────────────────────────────────────
+    openSections: getSidebarSectionsFromStorage(),
+
+    toggleSection(section) {
+      this.openSections[section] = !this.openSections[section];
+      saveSidebarSectionsToStorage(this.openSections);
+    },
+
+    // Auto-expand section containing current page
+    expandSectionForCurrentPage() {
+      const section = pageSectionMap[this.currentPage];
+      if (section && !this.openSections[section]) {
+        this.openSections[section] = true;
+        saveSidebarSectionsToStorage(this.openSections);
+      }
+    },
+
+    // ─────────────────────────────────────────────────────────────────
     // Page identifier - will be set by individual pages
+    // ─────────────────────────────────────────────────────────────────
     currentPage: ''
   }
 }