feat: add Claude Sonnet 4.6 model support

Adds claude-sonnet-4-6 to the model dropdown (marked Recommended),
pricing tables (frontend and backend), and context window limits
(1M token window).

src-tauri/src/stats.rs

@@ -86,8 +86,9 @@ impl ContextWarning {
 /// Get the context window limit (in tokens) for a given model
 fn get_context_window_limit(model: &str) -> u64 {
     match model {
-        // Claude 4.6 family - 200K standard (1M beta available via header)
+        // Claude 4.6 family
         "claude-opus-4-6" => 200_000,
+        "claude-sonnet-4-6" => 1_000_000, // 1M token context window
         // Claude 4.5 family - 200K standard context
         "claude-opus-4-5-20251101"
         | "claude-sonnet-4-5-20250929"
@@ -502,6 +503,7 @@ pub fn calculate_cost(
     let (input_price_per_million, output_price_per_million) = match model {
         // Current generation (Claude 4.6)
         "claude-opus-4-6" => (5.0, 25.0),
+        "claude-sonnet-4-6" => (3.0, 15.0),
 
         // Previous generation (Claude 4.5)
         "claude-opus-4-5-20251101" => (5.0, 25.0),