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

internal/content/assets/insertr.css

@@ -457,57 +457,7 @@ body:not(.insertr-edit-mode) .insertr-editing-hover::after {
     transform: none;
 }
 
-/* Active state for default style buttons */
-.insertr-style-btn.insertr-default-style.insertr-style-active {
-    background: var(--insertr-info);
-    border-color: var(--insertr-info);
-    color: white;
-    box-shadow: 0 2px 4px rgba(23, 162, 184, 0.3);
-}
 
-.insertr-style-btn.insertr-default-style.insertr-style-active .insertr-style-sample {
-    color: white;
-}
-
-/* Default formatting style buttons */
-.insertr-style-btn.insertr-default-style {
-    border-color: var(--insertr-info);
-    background: rgba(23, 162, 184, 0.1);
-    position: relative;
-}
-
-.insertr-style-btn.insertr-default-style:hover {
-    border-color: var(--insertr-info);
-    background: rgba(23, 162, 184, 0.2);
-    transform: translateY(-1px);
-    box-shadow: 0 2px 4px rgba(23, 162, 184, 0.3);
-}
-
-.insertr-style-btn.insertr-default-style:active {
-    transform: translateY(0);
-    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;
-}
-
-/* Small indicator for default styles */
-.insertr-style-btn.insertr-default-style::after {
-    content: '';
-    position: absolute;
-    top: 2px;
-    right: 2px;
-    width: 6px;
-    height: 6px;
-    background: var(--insertr-info);
-    border-radius: 50%;
-    opacity: 0.7;
-}
 
 /* Editor components */
 .insertr-simple-editor,