feat(themes): add decoupled CSS theme system with lightningcss bundling

- Add 6 syntax highlighting themes (dracula, gruvbox, nord, github)
- Rewrite css.rs to use lightningcss bundler for @import resolution
- Theme CSS is inlined at build time, producing single bundled output

docs/static/style.css

@@ -1,4 +1,6 @@
 /* sukr docs — clean documentation theme */
+@import "../../themes/default.css";
+/* Syntax highlighting theme - lightningcss inlines this at build time */
 
 :root {
   --bg: #0d1117;
@@ -313,128 +315,6 @@ pre code {
   padding: 0;
 }
 
-/* Syntax highlighting (tree-sitter classes)
- * Dracula-inspired color scheme with full coverage
- * All colors chosen to work well on dark backgrounds
- */
-
-/* Keywords: control flow, declarations - Pink */
-.hl-keyword {
-  color: #ff79c6;
-}
-
-/* Strings and literals - Yellow */
-.hl-string,
-.hl-string-special {
-  color: #f1fa8c;
-}
-
-.hl-string-escape {
-  color: #ffb86c;
-}
-
-.hl-string-special-path,
-.hl-string-special-uri {
-  color: #8be9fd;
-  text-decoration: underline;
-}
-
-/* Functions and methods - Green */
-.hl-function,
-.hl-function-builtin {
-  color: #50fa7b;
-}
-
-/* Comments - Muted blue-gray */
-.hl-comment {
-  color: #6272a4;
-  font-style: italic;
-}
-
-/* Numbers and constants - Purple */
-.hl-number,
-.hl-constant,
-.hl-constant-builtin {
-  color: #bd93f9;
-}
-
-/* Operators - Pink (same as keywords for visual consistency) */
-.hl-operator {
-  color: #ff79c6;
-}
-
-/* Punctuation - Subtle muted */
-.hl-punctuation,
-.hl-punctuation-bracket,
-.hl-punctuation-delimiter {
-  color: var(--fg-muted);
-}
-
-.hl-punctuation-special {
-  color: #ff79c6;
-}
-
-/* Types - Cyan */
-.hl-type,
-.hl-type-builtin {
-  color: #8be9fd;
-}
-
-/* Variables - Light foreground */
-.hl-variable {
-  color: var(--fg);
-}
-
-.hl-variable-builtin {
-  color: #ffb86c;
-}
-
-.hl-variable-parameter {
-  color: #ffb86c;
-  font-style: italic;
-}
-
-/* Properties and attributes - Cyan */
-.hl-property,
-.hl-attribute {
-  color: #8be9fd;
-}
-
-/* Constructors - Green */
-.hl-constructor {
-  color: #50fa7b;
-}
-
-/* HTML/XML tags - Pink */
-.hl-tag {
-  color: #ff79c6;
-}
-
-/* Embedded/escape content - Orange */
-.hl-embedded,
-.hl-escape {
-  color: #ffb86c;
-}
-
-/* Markdown-specific text highlighting */
-.hl-text-title {
-  color: #ff79c6;
-  font-weight: bold;
-}
-
-.hl-text-literal {
-  color: #8be9fd;
-}
-
-.hl-text-uri {
-  color: #8be9fd;
-  text-decoration: underline;
-}
-
-.hl-text-reference {
-  color: #bd93f9;
-}
-
 /* Mermaid diagrams */
 .mermaid-diagram {
   margin: 1.5rem 0;