Add AI Studio with agentic system, SnapGen video gen, Perchance image gen

Features:
- AI Studio module with 4 tabs: Video Generation, Image Generation, Lesson Plan, AI Agent Chat
- SnapGen AI API integration for video lesson generation (https://api.snapgen.ai/uapi/v1/generate)
  - Educational prompt enhancement with subject/topic/language context
  - Async polling for video generation status
  - Fallback to z-ai-web-dev-sdk if SnapGen not configured
- Perchance AI API integration for image generation (https://perchance.org/text-to-image-plugin)
  - Multiple styles: realistic, cartoon, watercolor, pixel, sketch, 3D
  - Fallback to z-ai-web-dev-sdk if Perchance not configured
- Agentic AI chat with teacher assistant context
- AI lesson plan generation

Fixes:
- Fix duplicate close button in onboarding tour (removed default Dialog close, kept custom one)
- Add SnapGen and Perchance fields to AISettings model

Navigation:
- AI Studio added to Insights section (teachers) and Student nav
- Digital Notebook confirmed available for both students and teachers

prisma/schema.prisma

@@ -2430,6 +2430,12 @@ model AISettings {
   aiMaxRequestsPerDay   Int      @default(50) // Student daily limit
   aiHelperMode          String   @default("guided") // guided, full, restricted
   aiSystemPrompt        String?  // Custom system prompt for AI chat
+  // SnapGen AI (Video generation)
+  snapgenEnabled       Boolean  @default(false)
+  snapgenApiKey        String?
+  // Perchance AI (Image generation)
+  perchanceEnabled     Boolean  @default(false)
+  perchanceApiKey      String?
   createdAt             DateTime @default(now())
   updatedAt             DateTime @updatedAt