feat(ui): update hero button text and enhance welcome screen

- Change primary hero button text from "Start Learning" to "Start Vibe Together!" across all language versions to align with project branding
- Update online reading links in README files to point to /welcome.html for better user onboarding
- Enhance WelcomeScreen.vue with improved animation timing and spacing
- Add dynamic SVG wordmark to homepage hero section for visual appeal
- Implement animated color transition for top promo bar
- Remove testing guidelines comment in AGENTS.md as placeholder
- Adjust feature card styling with increased min-height and icon sizes

docs/ar-sa/index.md

@@ -16,7 +16,7 @@ hero:
     - هكذا يجب أن تكون البرمجة.
   actions:
     - theme: brand
-      text: ابدأ التعلم
+      text: ابدأ vibe معًا!
       link: /ar-sa/stage-0/
     - theme: alt
       text: مخطط الدورة