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/pages/HelpGuide.tsx

@@ -0,0 +1,33 @@
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+import { BookOpen, Construction } from 'lucide-react';
+
+const HelpGuide: React.FC = () => {
+  const { t } = useTranslation();
+
+  return (
+    <div className="p-8 max-w-4xl mx-auto">
+      <div className="mb-8">
+        <h1 className="text-2xl font-bold text-gray-900 dark:text-white flex items-center gap-3">
+          <BookOpen className="text-brand-600" />
+          {t('help.guide.title', 'Platform Guide')}
+        </h1>
+        <p className="text-gray-500 dark:text-gray-400 mt-2">
+          {t('help.guide.subtitle', 'Learn how to use SmoothSchedule effectively')}
+        </p>
+      </div>
+
+      <div className="bg-white dark:bg-gray-800 rounded-xl border border-gray-200 dark:border-gray-700 p-12 text-center">
+        <Construction size={64} className="mx-auto text-gray-300 dark:text-gray-600 mb-4" />
+        <h2 className="text-xl font-semibold text-gray-900 dark:text-white mb-2">
+          {t('help.guide.comingSoon', 'Coming Soon')}
+        </h2>
+        <p className="text-gray-500 dark:text-gray-400 max-w-md mx-auto">
+          {t('help.guide.comingSoonDesc', 'We are working on comprehensive documentation to help you get the most out of SmoothSchedule. Check back soon!')}
+        </p>
+      </div>
+    </div>
+  );
+};
+
+export default HelpGuide;