PWA: installable app (web/win/android/ios) + download/help page + push notifications

- manifest.webmanifest + service worker (offline shell + push + notificationclick) + PNG icons (192/512/apple) + iOS meta + SW registration → installable everywhere
- /Download page: per-OS install help (web/windows/android/ios), install button (beforeinstallprompt), 'enable notifications' flow, usage guide, Bazaar/TWA note; nav + footer links
- Web Push foundation: WebPushSubscription entity + /push/subscribe (stores), VAPID + push settings in /Admin/Settings, on-device local notification; server broadcast documented (WebPush via Nexus)
- docs/PWA-TWA.md: VAPID keygen, server-push wiring, Bubblewrap→Cafe Bazaar + assetlinks steps
- Verified: manifest/sw/icons served, download page, subscribe stores (200), layout wired

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/JobsMedical.Web/Pages/Download.cshtml.cs

@@ -0,0 +1,20 @@
+using JobsMedical.Web.Services.Scraping;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace JobsMedical.Web.Pages;
+
+public class DownloadModel : PageModel
+{
+    private readonly SettingsService _settings;
+    public DownloadModel(SettingsService settings) => _settings = settings;
+
+    public string? VapidPublicKey { get; private set; }
+    public bool PushReady { get; private set; }
+
+    public async Task OnGetAsync()
+    {
+        var s = await _settings.GetAsync();
+        VapidPublicKey = s.VapidPublicKey;
+        PushReady = s.PushEnabled && !string.IsNullOrWhiteSpace(s.VapidPublicKey);
+    }
+}