refactor: add full documentation and fix theme loading robustness

- Add JSDoc comments across frontend and backend modules
- Improve code readability and maintainability with consistent documentation style
- Document Electron IPC handlers, UI utilities, and theme system
- Add missing function/class documentation in theme loader and app services
- Minor structural improvements and cleanup across Electron main process modules
- Minor fixes on variable names

public/script/appVersion.js

@@ -1,8 +1,21 @@
-async function versionLabel(){
-    const appVersion = await window.electronAPI.getVersion();
+/**
+ * Retrieves the application version from the Electron main process
+ * and displays it in the UI version badge (bottom-right corner).
+ *
+ * This is mainly used for debugging and build identification.
+ */
+async function versionLabel() {
+  const appVersion = await window.electronAPI.getVersion();
 
-    // Write in front the app version for debugging (bottom right)
-    document.getElementById("version-badge").textContent = `v${appVersion}`;
-};
+    /**
+     * UI element displaying the current application version.
+     * Updated at runtime after IPC call resolves.
+     */
+    const versionBadge = document.getElementById("version-badge");
+
+    if (!versionBadge) return;
+
+    versionBadge.textContent = `v${appVersion}`;
+}
 
 versionLabel();