feat(classes): admin manage pages + sidebar

/classes/manage (guarded by classes.manage) lists classes with inline
create/edit, archive/restore, delete (refused once sessions exist), an
optional grants-certification picker and a generic payment link, plus
per-class session scheduling with cancel/delete. Sidebar gets a "Manage
classes" link in the Admin block. The member-facing catalog/signup is
the next phase.

components/app-sidebar.tsx

@@ -14,7 +14,7 @@ import {
   Menu, X, LayoutDashboard, ListChecks, FolderKanban, Settings2, Wrench,
   MessageSquare, Users, CreditCard, BookUser, Download, LogOut,
   Vote, ShieldAlert, ScrollText, LineChart, UserCircle, UserSearch,
-  MessagesSquare, SlidersHorizontal, ClipboardList, FileText, Award, BadgeCheck,
+  MessagesSquare, SlidersHorizontal, ClipboardList, FileText, Award, BadgeCheck, GraduationCap,
 } from 'lucide-react'
 
 interface NavLinkProps {
@@ -148,6 +148,7 @@ export function AppSidebar({ member, roleName, taskBadge = 0, commsBadge = 0, pa
             <NavLink href="/customize" label="Customize" icon={SlidersHorizontal} active={isActive('/customize')} onClick={onNav} />
             <NavLink href="/forms" label="Forms & waivers" icon={ClipboardList} active={isActive('/forms')} onClick={onNav} />
             <NavLink href="/certifications" label="Certifications" icon={Award} active={isActive('/certifications')} onClick={onNav} />
+            <NavLink href="/classes/manage" label="Manage classes" icon={GraduationCap} active={isActive('/classes/manage')} onClick={onNav} />
             <NavLink href="/import" label="Import / Sync" icon={Download} active={isActive('/import')} onClick={onNav} />
             <NavLink href="/settings" label="Settings" icon={Settings2} active={isActive('/settings')} onClick={onNav} />
           </>