webui: remove unused language property from Monaco editor

Remove dead code - the language property was never set when the component
is used, so it always defaulted to 'javascript' and was never meaningful.

The getLanguageForFile method now uses a simpler fallback:
langMap[extension] || 'plaintext'

Since filenames always have extensions in practice, this removes an
unnecessary intermediate fallback that was never actually used.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s16c137c6267b58a5k
diff --git a/webui/src/web-components/sketch-monaco-view.ts b/webui/src/web-components/sketch-monaco-view.ts
index c58ac76..a1b948f 100644
--- a/webui/src/web-components/sketch-monaco-view.ts
+++ b/webui/src/web-components/sketch-monaco-view.ts
@@ -70,7 +70,6 @@
   editableRight?: boolean;
   private container: Ref<HTMLElement> = createRef();
   editor?: monaco.editor.IStandaloneDiffEditor;
-  @property() language?: string = "javascript";
 
   // Save state properties
   @state() private saveState: "idle" | "modified" | "saving" | "saved" = "idle";
@@ -545,7 +544,7 @@
       md: "markdown",
       go: "go",
     };
-    return langMap[extension] || this.language || "plaintext";
+    return langMap[extension] || "plaintext";
   }
 
   /**