feat: replace Tone Analysis with Explain - simplifies highlighted text in plain language

entrypoints/background.ts

@@ -53,11 +53,10 @@ function getSystemPrompt(action: string): string {
       'You are an experienced writer. Expand the provided text with more detail, context, and supporting points. ' +
       'Make it richer and more informative while staying on topic. ' +
       'Return ONLY the expanded text.',
-    tone:
-      'You are a writing coach. Analyze the tone of the provided text. ' +
-      'Describe the tone characteristics (e.g. formal, casual, aggressive, passive, confident, etc.) ' +
-      'and note any issues like passive voice, wordiness, or emotional bias. ' +
-      'Return a brief, clear analysis — no rewriting, no extra commentary.',
+    explain:
+      'You are a helpful teacher. Explain the following text in simple, easy-to-understand language. ' +
+      'Break down complex terms, jargon, or concepts so anyone can understand. ' +
+      'Be concise but clear. Return only the explanation, no extra commentary.',
   };
   return prompts[action] ?? prompts.grammar;
 }
@@ -411,8 +410,8 @@ export default defineBackground(() => {
       contexts: ['selection'],
     });
     chrome.contextMenus.create({
-      id: 'lexai-tone',
-      title: '⚡ LexAI: Analyze Tone',
+      id: 'lexai-explain',
+      title: '⚡ LexAI: Explain',
       contexts: ['selection'],
     });
   });