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/custompath.js

@@ -1,9 +1,24 @@
+/**
+ * Initializes the download path system once the UI is ready.
+ *
+ * Responsibilities:
+ * - Restore last known user-selected download path (localStorage)
+ * - Validate it against backend rules
+ * - Synchronize with backend authoritative path
+ * - Setup folder selection UI interaction
+ */
 window.addEventListener("DOMContentLoaded", async () => {
   const savePathElem = document.getElementById("savePath");
   const form = document.getElementById("downloadForm");
 
-  // input caché = DERNIER chemin validé par le back
+  /**
+   * Hidden form field storing the validated download path
+   * used during download submission.
+   *
+   * Created dynamically if not already present in DOM.
+   */
   let hidden = document.getElementById("savePathInput");
+  
   if (!hidden) {
     hidden = document.createElement("input");
     hidden.type = "hidden";
@@ -12,6 +27,17 @@ window.addEventListener("DOMContentLoaded", async () => {
     form.appendChild(hidden);
   }
 
+  /**
+   * Applies a validated download path received from backend.
+   *
+   * Updates:
+   * - UI display (visible path)
+   * - tooltip (full path)
+   * - hidden form input (used for submission)
+   * - localStorage cache (UX persistence only, not authoritative)
+   *
+   * @param {string} path - validated absolute download path
+   */
   async function applyPathFromBack(path) {
     savePathElem.textContent = path;
     savePathElem.title = path; 
@@ -19,6 +45,14 @@ window.addEventListener("DOMContentLoaded", async () => {
     localStorage.setItem("customDownloadPath", path); // UX only
   }
 
+  /**
+   * Loads the initial download path on application startup.
+   *
+   * Priority order:
+   * 1. cached localStorage value (UX only, not trusted)
+   * 2. backend validated version of cached path
+   * 3. backend default fallback path if validation fails
+   */
   async function loadInitialPath() {
     // On affiche ce que le user a vu la dernière fois (UX)
     const cached = localStorage.getItem("customDownloadPath");
@@ -42,13 +76,16 @@ window.addEventListener("DOMContentLoaded", async () => {
 
   await loadInitialPath();
 
-  document
-    .getElementById("changePath")
-    .addEventListener("click", async () => {
+  /**
+   * Opens system folder selector and updates download path.
+   *
+   * The selected folder is validated by Electron backend
+   * before being applied to the UI and persisted.
+   */
+  document.getElementById("changePath").addEventListener("click", async () => {
       try {
         // selectDownloadFolder already returns a validated path
-        const validatedPath =
-          await window.electronAPI.selectDownloadFolder();
+        const validatedPath = await window.electronAPI.selectDownloadFolder();
 
         if (!validatedPath) return; // cancelled