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

app/pathValidator.js

@@ -2,28 +2,77 @@ const fs = require("fs");
 const path = require("path");
 const { logger } = require("../server/logger");
 
+/**
+ * Cached resolved default download path.
+ * Initialized lazily on first access to avoid unnecessary imports.
+ */
 let _defaultPath = null;
 
+/**
+ * Returns the application's default download directory.
+ *
+ * This value is lazily loaded and cached after first call.
+ * The path originates from server configuration helpers.
+ *
+ * @returns {string} Absolute default download path
+ */
 function getDefaultDownloadPath() {
+
   if (!_defaultPath) {
+
+    /**
+     * Lazy-loaded dependency providing the configured default download folder.
+     * Required only when function is executed to reduce startup cost.
+     */
     const { defaultDownloadFolder } = require("../server/helpers/path.helpers");
+
     _defaultPath = defaultDownloadFolder;
+
   }
   return _defaultPath;
 }
 
+/**
+ * Validates and sanitizes a user-provided download path.
+ *
+ * Steps performed:
+ * - Resolves absolute real filesystem path
+ * - Normalizes symbolic links via realpathSync
+ * - Checks path safety rules (blocks system-critical directories)
+ * - Falls back to default path if input is empty
+ *
+ * @param {string} userPath - User-provided directory path
+ * @returns {string} Sanitized absolute path
+ * @throws Error If path is invalid, unsafe, or inaccessible
+ */
 function validateDownloadPath(userPath) {
+  
+  /**
+   * Lazy-loaded helper used to enforce safe directory constraints.
+   * Prevents writes to protected system folders.
+   */
   const { isSafePath } = require("../server/helpers/validation.helpers");
 
   if (!userPath) return getDefaultDownloadPath();
 
   try {
+
+    /**
+     * Resolves filesystem symlinks to ensure canonical absolute path.
+     * Required to prevent path traversal or alias bypass.
+     */
     const resolved = fs.realpathSync(path.resolve(userPath));
     if (!isSafePath(resolved)) {
       throw new Error("Path not allowed: system folders are blocked!");
     }
     return resolved;
-  } catch (err) {
+  }
+  catch (err) {
+    
+    /**
+     * Handles invalid filesystem paths, permissions issues, or resolution failures.
+     * Logs diagnostic information before propagating a sanitized error.
+     */
     logger.error(`Invalid download path: ${userPath} — ${err.message}`);
     throw new Error(`Invalid or inaccessible path: ${err.message}`);
   }