Unify all formatting buttons to use consistent three-layer architecture

- Remove .insertr-default-style special styling (blue background, border, dot indicator)
- Make ALL buttons use .insertr-style-preview with unified appearance
- Maintain authentic style previews in isolated .insertr-style-sample layer
- Bold buttons show bold text, .brand buttons show green uppercase text, etc.
- Eliminate visual inconsistency between semantic and detected style buttons
- Simplify CSS by removing ~50 lines of duplicate button styling
- Provide consistent professional toolbar appearance across all formatting options

lib/src/ui/style-aware-editor.js

@@ -639,12 +639,12 @@ export class StyleAwareEditor {
     applyStyleButtonStyling(button, styleSample, styleInfo) {
         if (!styleInfo) return;
 
-        // Apply styling based on whether this is a detected style or default
+        // ALL buttons use the same unified three-layer architecture
+        button.classList.add('insertr-style-preview');
+
+        // Apply appropriate preview styling to the isolated sample
         if (styleInfo.isDefault) {
-            // Default style - apply semantic styling
-            button.classList.add('insertr-default-style');
-            
-            // Add semantic styling for default elements
+            // Default semantic formatting - apply styling to sample
             if (styleInfo.tagName === 'strong') {
                 styleSample.style.fontWeight = 'bold';
             } else if (styleInfo.tagName === 'em') {
@@ -657,10 +657,7 @@ export class StyleAwareEditor {
             // Add helpful description to title
             button.title = `${styleInfo.name}: ${styleInfo.description || 'Default formatting option'}`;
         } else if (styleInfo.element && styleInfo.classes && styleInfo.classes.length > 0) {
-            // Detected style - apply original classes to style sample
-            button.classList.add('insertr-style-preview');
-            
-            // Add the detected classes to the style sample (isolated preview)
+            // Detected style - apply original classes to style sample (isolated preview)
             styleInfo.classes.forEach(className => {
                 styleSample.classList.add(className);
             });
@@ -668,7 +665,6 @@ export class StyleAwareEditor {
             button.title = `Apply ${styleInfo.classes.join(' ')} styling`;
         } else {
             // No meaningful styles detected - use fallback
-            button.classList.add('insertr-style-preview');
             styleSample.classList.add('insertr-fallback-style');
         }
     }