refactor(assets): PWA improvements and app config snake_case migration (#790)

- Service worker: versioned cache name, navigation preload, stale-while-revalidate for images
- New PWAModule extracted from MiscModule with async registration and update detection
- SW update notification toast in Hugo template with i18n support (16 languages)
- Web app manifest template (site.webmanifest) generated from [params.app] config
- [params.app] keys migrated to snake_case: name, short_name, pwa, no_favicon, svg_favicon, mask_color, tile_color, theme_color
- enablePWA moved from [params] to [params.app].pwa
- theme_color no longer accepts single-value, only {light, dark} map
- EventBus: new fixit:sw-update event
- SCSS: new _sw-toast.scss widget for update notification

layouts/_partials/base/widgets.html

@@ -135,6 +135,17 @@
   {{- /* Custom widgets */ -}}
   {{- block "custom-widgets" . }}{{ end -}}
 
+  {{- /* Service Worker Update Notification */ -}}
+  {{- if .Site.Params.app.pwa -}}
+    <div class="sw-update-notification">
+      <div class="sw-update-content">
+        <p class="sw-update-title">{{ T "serviceWorker.updateTitle" }}</p>
+        <p class="sw-update-text">{{ T "serviceWorker.updateText" }}</p>
+      </div>
+      <button type="button" class="sw-update-btn">{{ T "assets.refresh" }}</button>
+    </div>
+  {{- end -}}
+
   <noscript>
     <div class="noscript-warning">{{ T "baseof.noscript" }}</div>
   </noscript>