Fix: System-wide type hardening and V3 API alignment

src/lib/app_components/e_app_codemirror_v5.svelte

@@ -31,7 +31,7 @@
         content = 'test test test test',
         new_content = $bindable(),
         editorView = $bindable(), // Exposed for external control
-        theme_mode = 'light',
+        theme_mode = $bindable('light'),
         extensions = [],
         editable = true,
         readonly = false,
@@ -105,12 +105,17 @@
             ...extensions // Add any custom extensions passed in props
         ].filter(Boolean);
 
+        if (!editor_element) {
+            console.error('Editor element not found.');
+            return;
+        }
+
         editorView = new cm_modules.EditorView({
             state: cm_modules.EditorState.create({
                 doc: content,
                 extensions: editor_extensions
             }),
-            parent: editor_element
+            parent: editor_element as HTMLElement
         });
     }