feat: Improve quote format with guillemets and styling

- Changed quote format to: @[Username]: »quoted text«
- Added processQuotes() to detect and style »text« patterns
- Quote styling: italic, gray background, left border
- Different styling for own messages vs others

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/static/js/app.js

@@ -782,7 +782,7 @@ function quoteTo(username, content) {
         quotedText = truncated + '...';
     }
 
-    input.value = `@[${username}] "${quotedText}" `;
+    input.value = `@[${username}]: »${quotedText}« `;
     updateCharCounter();
     input.focus();
 }