feat: coloration syntaxique via CSS classes (themeable)

Remplace les styles inline syntect par des classes CSS. Le CSS est
généré côté Rust (base16-ocean.dark) et injecté au démarrage via
get_syntax_highlight_css. Les thèmes peuvent surcharger .syntax-highlighting
et les classes de tokens. Supprime le lien CDN highlight.js inutilisé.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/src/commands/render.rs

@@ -1,22 +1,25 @@
 use crate::application::{file_service, render_service};
-use crate::infrastructure::comrak_renderer::{ComrakPreviewRenderer, ComrakRenderer};
+use crate::infrastructure::comrak_renderer::{
+    syntax_css_for_theme, ComrakPreviewRenderer, ComrakRenderer,
+};
 
 #[tauri::command]
 pub fn render_markdown(content: String) -> String {
-    let renderer = ComrakPreviewRenderer {
-        theme: "base16-ocean.dark".to_string(),
-    };
+    let renderer = ComrakPreviewRenderer;
     render_service::render_string(&renderer, &content)
 }
 
 #[tauri::command]
 pub fn convert_file(path: String) -> Result<String, String> {
-    let renderer = ComrakRenderer {
-        theme: "InspiredGitHub".to_string(),
-    };
+    let renderer = ComrakRenderer;
     render_service::render_file(&renderer, &path)
 }
 
+#[tauri::command]
+pub fn get_syntax_highlight_css() -> String {
+    syntax_css_for_theme("base16-ocean.dark")
+}
+
 #[tauri::command]
 pub fn list_md_files(dir: String) -> Result<Vec<String>, String> {
     file_service::list_markdown_files(&dir)
@@ -41,7 +44,7 @@ mod tests {
     #[test]
     fn render_markdown_code_block() {
         let html = render_markdown("```rust\nfn main() {}\n```\n".to_string());
-        assert!(html.contains("<code") || html.contains("<pre>"));
+        assert!(html.contains("syntax-highlighting"));
     }
 
     #[test]
@@ -80,6 +83,18 @@ mod tests {
         assert!(result.is_err());
     }
 
+    #[test]
+    fn get_syntax_highlight_css_returns_non_empty() {
+        let css = get_syntax_highlight_css();
+        assert!(!css.is_empty());
+    }
+
+    #[test]
+    fn get_syntax_highlight_css_contains_wrapper_class() {
+        let css = get_syntax_highlight_css();
+        assert!(css.contains(".syntax-highlighting {"));
+    }
+
     #[test]
     fn list_md_files_sorted() {
         let dir = tmpdir("pena_list_sorted");