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/components/PaymentSettingsSection.tsx

@@ -235,7 +235,7 @@ const PaymentSettingsSection: React.FC<PaymentSettingsSectionProps> = ({ busines
                 </ul>
                 <div className="flex flex-wrap gap-3">
                   <button
-                    onClick={() => navigate('/settings/billing')}
+                    onClick={() => navigate('/dashboard/settings/billing')}
                     className="flex items-center gap-2 px-5 py-2.5 text-sm font-medium text-white bg-purple-600 rounded-lg hover:bg-purple-700 transition-colors"
                   >
                     <ArrowUpRight size={16} />