Fix whitespace issue in default formatting buttons

- Add margin/padding reset to .insertr-style-sample to prevent browser defaults
- Remove unused .insertr-default-preview CSS that was causing confusion
- Ensure consistent spacing across all button types (default vs detected styles)
- Clean up legacy CSS classes that are no longer used with three-layer architecture

internal/content/assets/insertr.css

@@ -417,6 +417,10 @@ body:not(.insertr-edit-mode) .insertr-editing-hover::after {
     text-overflow: ellipsis;
     white-space: nowrap;
     
+    /* Reset any browser default margins/padding that could cause inconsistency */
+    margin: 0;
+    padding: 0;
+    
     /* All other styling (color, weight, transform, decoration, etc.) 
        comes from applied classes - no interference */
 }
@@ -424,6 +428,9 @@ body:not(.insertr-edit-mode) .insertr-editing-hover::after {
 /* Fallback styling when no meaningful classes are detected */
 .insertr-style-sample.insertr-fallback-style {
     color: var(--insertr-text-primary);
+    /* Ensure consistent text appearance for action buttons */
+    font-size: inherit;
+    font-weight: inherit;
 }
 
 /* Hover state for preview buttons - subtle visual feedback */
@@ -488,13 +495,7 @@ body:not(.insertr-edit-mode) .insertr-editing-hover::after {
     box-shadow: 0 1px 2px rgba(23, 162, 184, 0.2);
 }
 
-/* Default preview content styling */
-.insertr-default-preview {
-    font-size: var(--insertr-font-size-sm);
-    font-weight: 500;
-    padding: var(--insertr-spacing-xs) var(--insertr-spacing-sm);
-    display: inline-block;
-}
+/* Legacy class - no longer used with three-layer button architecture */
 
 /* Small indicator for default styles */
 .insertr-style-btn.insertr-default-style::after {