feat: add support for Anthropic Claude 4 models and update SDK to v1.2.0

CHANGES
- Upgrade `anthropic-sdk-go` dependency to version `v1.2.0`.
- Integrate new Anthropic Claude 4 Opus and Sonnet models.
- Remove deprecated Claude 2.0 and 2.1 models from list.
- Adjust model type casting for `anthropic-sdk-go v1.2.0` compatibility.
- Refresh README: announce Claude 4, update date, fix links.

go.mod

@@ -6,7 +6,7 @@ toolchain go1.24.2
 
 require (
 	github.com/anaskhan96/soup v1.2.5
-	github.com/anthropics/anthropic-sdk-go v0.2.0-beta.3
+	github.com/anthropics/anthropic-sdk-go v1.2.0
 	github.com/atotto/clipboard v0.1.4
 	github.com/gabriel-vasile/mimetype v1.4.9
 	github.com/gin-gonic/gin v1.10.0