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/UpgradePrompt.tsx

@@ -51,7 +51,7 @@ const BannerPrompt: React.FC<{ feature: FeatureKey; showDescription: boolean }>
           </p>
         )}
         <Link
-          to="/settings/billing"
+          to="/dashboard/settings/billing"
           className="inline-flex items-center gap-2 px-4 py-2 rounded-lg bg-gradient-to-r from-amber-500 to-orange-500 text-white font-medium hover:from-amber-600 hover:to-orange-600 transition-all shadow-md hover:shadow-lg"
         >
           <Crown className="w-4 h-4" />
@@ -97,7 +97,7 @@ const OverlayPrompt: React.FC<{
             {FEATURE_DESCRIPTIONS[feature]}
           </p>
           <Link
-            to="/settings/billing"
+            to="/dashboard/settings/billing"
             className="inline-flex items-center gap-2 px-6 py-3 rounded-lg bg-gradient-to-r from-amber-500 to-orange-500 text-white font-medium hover:from-amber-600 hover:to-orange-600 transition-all shadow-md hover:shadow-lg"
           >
             <Crown className="w-5 h-5" />