feat: add native code block feature with Prism highlighting and editor integration

- Implemented custom code block system for frontend and editors
- Integrated Prism.js for syntax highlighting (YAML + HTML support)
- Added copy-to-clipboard functionality with hover-based UI
- Introduced custom Gutenberg block for code input
- Added Classic Editor button for quick code insertion
- Implemented server-side rendering via the_content filter
- Added dedicated styling (code.css) with Dracula-inspired theme
- Added editor preview styling (editor.css) for visual consistency
- Ensured accessibility and keyboard support for copy button
- Optimized asset loading and versioning using filemtime()

This feature provides a lightweight, theme-native alternative to external code highlighting plugins.

assets/css/editor.css

@@ -0,0 +1,116 @@
+/**
+ * Editor styles
+ *
+ * Scoped styles for Gutenberg and Classic Editor.
+ */
+
+/* -------------------------------------------------------------------------
+   Editor base
+------------------------------------------------------------------------- */
+
+.editor-styles-wrapper,
+.mce-content-body {
+  font-family: var(--secondary-font);
+  line-height: 1.7;
+}
+
+/* -------------------------------------------------------------------------
+   Shared code block preview
+------------------------------------------------------------------------- */
+
+.editor-styles-wrapper .ztfr-code,
+.editor-styles-wrapper pre.language-yaml,
+.mce-content-body pre.language-yaml {
+  position: relative;
+  background-color: #282a36;
+  color: #f8f8f2;
+  border: 1px solid rgba(248, 248, 242, 0.08);
+  border-radius: 6px;
+  padding: 1rem 1.2rem;
+  margin: 1rem 0;
+  max-width: 100%;
+  overflow-x: auto;
+  font-family: var(--secondary-font);
+  font-size: 0.95rem;
+  line-height: 1.7;
+  box-sizing: border-box;
+}
+
+/* -------------------------------------------------------------------------
+   Gutenberg preview
+------------------------------------------------------------------------- */
+
+.editor-styles-wrapper .ztfr-code pre,
+.editor-styles-wrapper .ztfr-code code {
+  background: transparent;
+  color: inherit;
+}
+
+.editor-styles-wrapper .ztfr-code pre.language-yaml,
+.editor-styles-wrapper .ztfr-code code.language-yaml {
+  background: transparent !important;
+  padding: 0 !important;
+  margin: 0 !important;
+  white-space: pre-wrap !important;
+  word-break: break-word !important;
+  overflow-wrap: break-word !important;
+}
+
+.editor-styles-wrapper .ztfr-code pre code {
+  display: block;
+  white-space: pre-wrap;
+  word-break: break-word;
+  overflow-wrap: break-word;
+}
+
+/* -------------------------------------------------------------------------
+   Classic Editor preview
+------------------------------------------------------------------------- */
+
+.mce-content-body pre.language-yaml {
+  white-space: pre-wrap;
+  word-break: break-word;
+  overflow-wrap: break-word;
+}
+
+.mce-content-body pre.language-yaml code.language-yaml {
+  display: block;
+  background: transparent;
+  color: inherit;
+  padding: 0;
+  margin: 0;
+  white-space: pre-wrap;
+  word-break: break-word;
+  overflow-wrap: break-word;
+}
+
+/* -------------------------------------------------------------------------
+   Editor label
+------------------------------------------------------------------------- */
+
+.editor-styles-wrapper .ztfr-code.is-editor-preview::before,
+.mce-content-body pre.language-yaml::before {
+  content: "YAML";
+  display: inline-block;
+  margin-bottom: 0.75rem;
+  padding: 0.2rem 0.45rem;
+  border-radius: 4px;
+  background: #44475a;
+  color: #f8f8f2;
+  font-size: 0.72rem;
+  line-height: 1;
+  letter-spacing: 0.03em;
+}
+
+/* -------------------------------------------------------------------------
+   Lists
+------------------------------------------------------------------------- */
+
+.editor-styles-wrapper ul,
+.editor-styles-wrapper ol,
+.mce-content-body ul,
+.mce-content-body ol {
+  margin-left: 0;
+  padding-left: 1.25em;
+  list-style-position: outside;
+}