Move tenant dashboard routes under /dashboard/ prefix

- Update App.tsx routes to use /dashboard/ prefix for all business user routes
- Add redirect from / to /dashboard for authenticated business users
- Update Sidebar.tsx navigation links with /dashboard/ prefix
- Update SettingsLayout.tsx settings navigation paths
- Update all help pages with /dashboard/help/ routes
- Update navigate() calls in components:
  - TrialBanner, PaymentSettingsSection, NotificationDropdown
  - BusinessLayout, UpgradePrompt, QuotaWarningBanner
  - QuotaOverageModal, OpenTicketsWidget, CreatePlugin
  - MyPlugins, PluginMarketplace, HelpTicketing
  - HelpGuide, Upgrade, TrialExpired
  - CustomDomainsSettings, QuotaSettings
- Fix hardcoded lvh.me URL in BusinessEditModal to use buildSubdomainUrl

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/settings/CustomDomainsSettings.tsx

@@ -134,7 +134,7 @@ const CustomDomainsSettings: React.FC = () => {
         {isCustomDomainLocked && (
           <div className="absolute inset-0 z-10 bg-white/70 dark:bg-gray-900/70 backdrop-blur-[2px] rounded-xl flex items-center justify-center">
             <Link
-              to="/settings/billing"
+              to="/dashboard/settings/billing"
               className="inline-flex items-center gap-2 px-6 py-3 bg-gradient-to-r from-amber-500 to-orange-500 text-white font-semibold rounded-lg shadow-lg hover:from-amber-600 hover:to-orange-600 transition-all"
             >
               <Lock size={18} />