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/dependencyCheck.js

@@ -2,9 +2,33 @@ const fs = require("fs");
 const { app, dialog } = require("electron");
 const { logger } = require("../server/logger");
 
+/**
+ * Verifies that all required native binaries exist before the app starts.
+ *
+ * This includes:
+ * - yt-dlp (download engine)
+ * - ffmpeg (media processing)
+ * - ffprobe (media inspection)
+ * - deno (JS runtime used by yt-dlp hooks)
+ *
+ * If any dependency is missing:
+ * - logs the error
+ * - shows a blocking Electron error dialog
+ * - exits the application
+ *
+ * @returns {boolean} true if all dependencies are present, false otherwise
+ */
 function checkNativeDependencies() {
+  /**
+   * Native dependency paths are resolved at runtime from the server helpers.
+   * This ensures correct resolution in both dev and packaged builds.
+   */
   const { binaryPaths } = require("../server/helpers/path.helpers");
 
+  /**
+   * List of required external executables used by the application runtime.
+   * Each entry defines a binary name and its resolved absolute path.
+   */
   const deps = [
     { name: "yt-dlp.exe",  path: binaryPaths.ytDlp },
     { name: "ffmpeg.exe",  path: binaryPaths.ffmpeg },
@@ -15,9 +39,16 @@ function checkNativeDependencies() {
   const missing = deps.filter(d => !fs.existsSync(d.path));
   if (missing.length === 0) return true;
 
+  /**
+   * Human-readable list of missing binaries for logging and user dialog display.
+   */
   const list = missing.map(d => d.name).join(", ");
   logger.error(`Missing dependencies: ${list}`);
 
+  /**
+   * Defers error dialog until Electron app is fully initialized.
+   * Required to safely display modal dialogs before quit.
+   */
   app.whenReady().then(() => {
     dialog.showErrorBox(
       "Missing dependencies",