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/clipboardPaste.js

@@ -1,35 +1,42 @@
-// Clipboard paste functionality for URL input
-document.addEventListener('DOMContentLoaded', () => {
-    const pasteBtn = document.getElementById('pasteBtn');
-    const urlInput = document.getElementById('UrlInput');
+/**
+ * Initializes clipboard paste functionality for the URL input field.
+ *
+ * When the user clicks the paste button, the system attempts to read
+ * the clipboard content and inject it into the URL input field.
+ * It also triggers an input event to notify any bound listeners.
+ */
+document.addEventListener("DOMContentLoaded", () => {
+  const pasteBtn = document.getElementById("pasteBtn");
+  const urlInput = document.getElementById("UrlInput");
 
-    if (pasteBtn && urlInput) {
-        pasteBtn.addEventListener('click', async () => {
-            try {
-                // Read text from clipboard
-                const text = await navigator.clipboard.readText();
-                
-                // Paste into the URL input
-                urlInput.value = text;
-                
-                // Trigger input event to ensure any listeners are notified
-                urlInput.dispatchEvent(new Event('input', { bubbles: true }));
-                
-                // Optional: Focus the input
-                urlInput.focus();
-                
-                // Visual feedback
-                pasteBtn.style.backgroundColor = 'var(--form-button-bg-hover-color)';
-                setTimeout(() => {
-                    pasteBtn.style.backgroundColor = '';
-                }, 200);
-                
-            } catch (err) {
-                console.error('Failed to read clipboard:', err);
-                
-                // Fallback: prompt user if clipboard API fails
-                alert('Unable to access clipboard. Please paste manually using Ctrl+V.');
-            }
-        });
+  if (!pasteBtn || !urlInput) return;
+
+  /**
+   * Handles clipboard paste action triggered by the paste button.
+   * Reads clipboard text and inserts it into the URL input field.
+   */
+  pasteBtn.addEventListener("click", async () => {
+    try {
+      const text = await navigator.clipboard.readText();
+
+      urlInput.value = text;
+
+      // Notify frameworks / listeners bound to input changes
+      urlInput.dispatchEvent(new Event("input", { bubbles: true }));
+
+      urlInput.focus();
+
+      // Temporary UI feedback on paste action
+      pasteBtn.style.backgroundColor = "var(--form-button-bg-hover-color)";
+      setTimeout(() => {
+        pasteBtn.style.backgroundColor = "";
+      }, 200);
+    } catch (err) {
+      console.error("Clipboard access failed:", err);
+
+      alert(
+        "Unable to access clipboard. Please paste manually using Ctrl+V."
+      );
     }
-});
+  });
+});