feat: Implement dynamic style preview buttons

- Add JavaScript-based style copying from original elements to toolbar buttons
- Use getComputedStyle() to dynamically apply color, font-weight, text-decoration, text-transform
- Preserve button clickability with protected backgrounds and hover states
- Support any CSS framework/custom styles without hardcoded mappings
- Add comprehensive documentation to TODO.md for future enhancements

Examples:
- 'Emphasis' button now shows red bold text (from .emph class)
- 'Highlight' button displays with style preview while remaining clickable
- 'Brand' button demonstrates text-transform and color changes

This provides intuitive visual feedback so users immediately understand
what each formatting button will do to their content.

internal/content/assets/insertr.css

@@ -380,6 +380,56 @@ body:not(.insertr-edit-mode) .insertr-editing-hover::after {
     transform: translateY(1px);
 }
 
+/* Style preview buttons - styled dynamically via JavaScript */
+.insertr-style-btn.insertr-style-preview {
+    /* Preserve button structure */
+    background: var(--insertr-bg-primary) !important;
+    border: 1px solid var(--insertr-border-color) !important;
+    border-radius: var(--insertr-border-radius) !important;
+    padding: var(--insertr-spacing-xs) var(--insertr-spacing-sm) !important;
+    font-size: var(--insertr-font-size-sm) !important;
+    cursor: pointer !important;
+    transition: var(--insertr-transition) !important;
+    
+    /* Button layout */
+    min-height: 28px;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    
+    /* Ensure button remains clickable */
+    position: relative;
+    
+    /* Styles will be applied dynamically via JavaScript */
+}
+
+/* Add subtle background to preview buttons to ensure they remain clickable-looking */
+.insertr-style-btn.insertr-style-preview::before {
+    content: '';
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    background: var(--insertr-bg-primary);
+    opacity: 0.9;
+    border-radius: inherit;
+    z-index: -1;
+}
+
+/* Hover state for preview buttons */
+.insertr-style-btn.insertr-style-preview:hover {
+    background: var(--insertr-bg-secondary) !important;
+    border-color: var(--insertr-text-muted) !important;
+    transform: none !important;
+}
+
+/* Active state for preview buttons */
+.insertr-style-btn.insertr-style-preview:active {
+    background: var(--insertr-border-color) !important;
+    transform: translateY(1px) !important;
+}
+
 /* Editor components */
 .insertr-simple-editor,
 .insertr-rich-editor,