feat(example): rebuild as comprehensive component showcase (#103)

Complete rebuild of the example app showcasing every major library feature:\n- Shell: all Sidebar sub-components, CommandBarTrigger, user avatar footer\n- Dashboard: stat cards, bar chart, activity feed, hooks demos, toasts\n- Data: invoice table with select-all, filters, row actions, formatters\n- Components: Button/Toggle/Input/Select/Switch/Slider/Alert/Badge/Progress/Skeleton/Dialog/Sheet/AlertDialog/DropdownMenu/Tooltip/Popover/Accordion/Tabs/Avatar/Separator/Breadcrumb\n- Admin: SignedIn/SignedOut/RoleGate/useHasPermission/useUser/useSession\n- Settings: 4-tab forms with useCopyToClipboard/useLocalStorage/AlertDialog

Author: jonmatum