feat: add Opus [1M] model option to model selector

Adds opus[1m] to the Claude model options alongside the existing sonnet[1m],
allowing users to select the Opus model with 1M context window.

Closes #579

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gumab

shared/modelConstants.js

@@ -18,6 +18,7 @@ export const CLAUDE_MODELS = {
     { value: "haiku", label: "Haiku" },
     { value: "opusplan", label: "Opus Plan" },
     { value: "sonnet[1m]", label: "Sonnet [1M]" },
+    { value: "opus[1m]", label: "Opus [1M]" },
   ],
 
   DEFAULT: "sonnet",