fix: show dynamic app version everywhere in UI

- Add get-app-version IPC handler in main.js (returns app.getVersion())
- Expose electronAPI.getAppVersion() in preload.js
- index.html: replace hardcoded v4.2.0 span with dynamic population
  from getAppVersion() in DOMContentLoaded
- welcome.js: accept appVersion param instead of hardcoded 4.1.0
- renderer.js: pass live version to createWelcomeContent()
- main.js about screen: use app.getVersion() instead of hardcoded 4.1.0
- Update stale @version 4.1.0 JSDoc comments to 4.3.0

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer.js

@@ -1,6 +1,6 @@
 /**
  * MarkdownConverter Renderer Process
- * @version 4.1.0
+ * @version 4.3.0
  */
 
 const { ipcRenderer } = require('electron');
@@ -1442,11 +1442,17 @@ class TabManager {
 let tabManager;
 let replPanel;
 
-document.addEventListener('DOMContentLoaded', () => {
+document.addEventListener('DOMContentLoaded', async () => {
     tabManager = new TabManager();
     const ReplPanel = getReplPanel();
     replPanel = new ReplPanel();
 
+    // Populate app version from main process
+    window.electronAPI.getAppVersion().then((version) => {
+      const el = document.getElementById('app-version-display');
+      if (el) el.textContent = `v${version}`;
+    });
+
     // Initialize ModalManager for all dialogs
     const findModal = new ModalManager('#find-dialog');
     const exportModal = new ModalManager('#export-dialog');
@@ -1613,7 +1619,8 @@ document.addEventListener('DOMContentLoaded', () => {
 
         // Set welcome content in the first tab's preview
         const recentFiles = JSON.parse(localStorage.getItem('recentFiles') || '[]');
-        const welcomeHtml = getCreateWelcomeContent()(recentFiles);
+        const appVersion = await window.electronAPI.getAppVersion().catch(() => '');
+        const welcomeHtml = getCreateWelcomeContent()(recentFiles, appVersion);
 
         const tab = tabManager.tabs.get(tabManager.activeTabId);
         if (tab) {