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/PluginMarketplace.tsx

@@ -605,7 +605,7 @@ const PluginMarketplace: React.FC = () => {
                     setShowDetailsModal(false);
                     setSelectedPlugin(null);
                     setShowCode(false);
-                    navigate('/plugins/my-plugins');
+                    navigate('/dashboard/plugins/my-plugins');
                   }}
                   className="flex items-center gap-2 px-4 py-2 bg-green-600 text-white rounded-lg hover:bg-green-700 transition-colors font-medium"
                 >
@@ -664,7 +664,7 @@ const PluginMarketplace: React.FC = () => {
                 onClick={() => {
                   setShowWhatsNextModal(false);
                   setSelectedPlugin(null);
-                  navigate('/tasks');
+                  navigate('/dashboard/tasks');
                 }}
                 className="w-full flex items-center gap-4 p-4 border border-gray-200 dark:border-gray-700 rounded-lg hover:border-blue-500 dark:hover:border-blue-500 hover:bg-blue-50 dark:hover:bg-blue-900/20 transition-colors text-left group"
               >
@@ -687,7 +687,7 @@ const PluginMarketplace: React.FC = () => {
                 onClick={() => {
                   setShowWhatsNextModal(false);
                   setSelectedPlugin(null);
-                  navigate('/plugins/my-plugins');
+                  navigate('/dashboard/plugins/my-plugins');
                 }}
                 className="w-full flex items-center gap-4 p-4 border border-gray-200 dark:border-gray-700 rounded-lg hover:border-purple-500 dark:hover:border-purple-500 hover:bg-purple-50 dark:hover:bg-purple-900/20 transition-colors text-left group"
               >