feat: Add comprehensive sandbox mode, public API system, and platform support

This commit adds major features for sandbox isolation, public API access, and platform support ticketing.

## Sandbox Mode
- Add sandbox mode toggle for businesses to test features without affecting live data
- Implement schema-based isolation for tenant data (appointments, resources, services)
- Add is_sandbox field filtering for shared models (customers, staff, tickets)
- Create sandbox middleware to detect and set sandbox mode from cookies
- Add sandbox context and hooks for React frontend
- Display sandbox banner when in test mode
- Auto-reload page when switching between live/test modes
- Prevent platform support tickets from being created in sandbox mode

## Public API System
- Full REST API for external integrations with businesses
- API token management with sandbox/live token separation
- Test tokens (ss_test_*) show full plaintext for easy testing
- Live tokens (ss_live_*) are hashed and secure
- Security validation prevents live token plaintext storage
- Comprehensive test suite for token security
- Rate limiting and throttling per token
- Webhook support for real-time event notifications
- Scoped permissions system (read/write per resource type)
- API documentation page with interactive examples
- Token revocation with confirmation modal

## Platform Support
- Dedicated support page for businesses to contact SmoothSchedule
- View all platform support tickets in one place
- Create new support tickets with simplified interface
- Reply to existing tickets with conversation history
- Platform tickets have no admin controls (no priority/category/assignee/status)
- Internal notes hidden for platform tickets (business can't see them)
- Quick help section with links to guides and API docs
- Sandbox warning prevents ticket creation in test mode
- Business ticketing retains full admin controls (priority, assignment, internal notes)

## UI/UX Improvements
- Add notification dropdown with real-time updates
- Staff permissions UI for ticket access control
- Help dropdown in sidebar with Platform Guide, Ticketing Help, API Docs, and Support
- Update sidebar "Contact Support" to "Support" with message icon
- Fix navigation links to use React Router instead of anchor tags
- Remove unused language translations (Japanese, Portuguese, Chinese)

## Technical Details
- Sandbox middleware sets request.sandbox_mode from cookies
- ViewSets filter data by is_sandbox field
- API authentication via custom token auth class
- WebSocket support for real-time ticket updates
- Migration for sandbox fields on User, Tenant, and Ticket models
- Comprehensive documentation in SANDBOX_MODE_IMPLEMENTATION.md

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

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

frontend/src/components/PlatformSidebar.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import { useTranslation } from 'react-i18next';
 import { Link, useLocation } from 'react-router-dom';
-import { LayoutDashboard, Building2, MessageSquare, Settings, Users, Shield } from 'lucide-react';
+import { LayoutDashboard, Building2, MessageSquare, Settings, Users, Shield, HelpCircle, Code } from 'lucide-react';
 import { User } from '../types';
 import SmoothScheduleLogo from './SmoothScheduleLogo';
 
@@ -77,6 +77,18 @@ const PlatformSidebar: React.FC<PlatformSidebarProps> = ({ user, isCollapsed, to
             </Link>
           </>
         )}
+
+        {/* Help Section */}
+        <div className="mt-8 pt-4 border-t border-gray-800">
+          <Link to="/help/ticketing" className={getNavClass('/help/ticketing')} title={t('nav.help', 'Help')}>
+            <HelpCircle size={18} className="shrink-0" />
+            {!isCollapsed && <span>{t('nav.help', 'Help')}</span>}
+          </Link>
+          <Link to="/help/api" className={getNavClass('/help/api')} title={t('nav.apiDocs', 'API Documentation')}>
+            <Code size={18} className="shrink-0" />
+            {!isCollapsed && <span>{t('nav.apiDocs', 'API Docs')}</span>}
+          </Link>
+        </div>
       </nav>
     </div>
   );