fix: ignore underscore-prefixed unused args in eslint

The compositionGuardPlugin added in 5d89a69 declares _view/_event
handler args that are intentionally unused, prefixed with _ per the
existing convention. no-unused-vars only had varsIgnorePattern, so
these args failed the lint step and broke CI on main. Add
argsIgnorePattern: '^_' to honor the prefix convention.

https://claude.ai/code/session_01KndTBoSS4psob1CTs9q5cs

Author: claude

frontend/eslint.config.js

@@ -23,7 +23,7 @@ export default defineConfig([
       },
     },
     rules: {
-      'no-unused-vars': ['error', { varsIgnorePattern: '^[A-Z_]' }],
+      'no-unused-vars': ['error', { varsIgnorePattern: '^[A-Z_]', argsIgnorePattern: '^_' }],
     },
   },
 ])