feat: add Happy New Year announcement banners across all sites

- Book: New Year greeting + navbar, TinyTorch, Kits, Newsletter
- Kits: New Year greeting + navbar, Kits intro, Textbook link
- Labs: New Year greeting + navbar, Labs coming 2026, Textbook link

book/quarto/config/announcement.yml

@@ -12,6 +12,7 @@ website:
     type: primary
     position: below-navbar
     content: |
-      🔥 **New Release:** TinyTorch ML framework. Don't import torch. [Build it →](https://mlsysbook.ai/tinytorch)<br>
+      🎉 **Happy New Year!** New navbar with dropdown menus. Try them out!<br>
+      🔥 **TinyTorch:** Build your own ML framework from scratch. [Start →](https://mlsysbook.ai/tinytorch)<br>
       📦 **Hardware Kits:** Arduino, Seeed & Raspberry Pi labs. [Explore →](https://mlsysbook.ai/kits)<br>
-      📬 **Newsletter:** ML Systems insights, tutorials & industry news. [Subscribe →](#subscribe)
+      📬 **Newsletter:** ML Systems insights & updates. [Subscribe →](#subscribe)

kits/config/announcement.yml

@@ -12,5 +12,6 @@ website:
     type: primary
     position: below-navbar
     content: |
+      🎉 **Happy New Year!** New navbar with dropdown menus. Try them out!<br>
       📦 **Hardware Kits:** Hands-on ML labs for Arduino, Seeed & Raspberry Pi. [Get started →](/contents/getting-started.html)<br>
       📚 **Textbook:** Read the ML Systems book. [Explore →](https://mlsysbook.ai/book)

labs/config/announcement.yml

@@ -12,5 +12,6 @@ website:
     type: primary
     position: below-navbar
     content: |
+      🎉 **Happy New Year!** New navbar with dropdown menus. Try them out!<br>
       🔮 **Labs:** Interactive ML simulations coming in 2026. [Learn more →](/)<br>
       📚 **Textbook:** Read the ML Systems book. [Explore →](https://mlsysbook.ai/book)