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/index.html

@@ -31,7 +31,7 @@
                 <span class="app-title">MarkdownConverter</span>
             </div>
             <div class="app-header-right">
-                <span class="app-version">v4.2.0</span>
+                <span class="app-version" id="app-version-display"></span>
             </div>
         </div>
         <div class="tab-bar" id="tab-bar" role="tablist" aria-label="Document tabs">