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

src/lib/i18n.ts

@@ -818,6 +818,7 @@ const de: TranslationDict = {
   'polish.nav_school_life': 'Schulleben',
   'polish.nav_insights': 'Erkenntnisse',
   'polish.nav_administration': 'Verwaltung',
+  'nav.ai-studio': 'KI-Studio',
   'polish.nav_setup': 'Einrichtung',
   'polish.nav_teaching': 'Unterricht',
   'polish.nav_analysis': 'Analyse',
@@ -7472,6 +7473,7 @@ const en: TranslationDict = {
   'polish.nav_school_life': 'School Life',
   'polish.nav_insights': 'Insights',
   'polish.nav_administration': 'Administration',
+  'nav.ai-studio': 'AI Studio',
   'polish.nav_setup': 'Setup',
   'polish.nav_teaching': 'Teaching',
   'polish.nav_analysis': 'Analysis',