feat: PWA + mobile-friendly layout

- Add vite-plugin-pwa: generates manifest, Workbox service worker, precaches app shell
- Generate icon-192, icon-512, apple-touch-icon PNGs from existing SVG logo
- Add theme-color, apple-mobile-web-app-capable, apple-touch-icon meta tags; viewport-fit=cover for notch support
- Mobile layout (< 768px): bottom tab bar (Library, Albums, Memories, Map, More) replaces top nav; sidebar becomes slide-in filter drawer with backdrop; bulk action bar clears bottom nav; safe-area insets throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/index.html

@@ -2,8 +2,14 @@
 <html lang="en">
   <head>
     <meta charset="UTF-8" />
-    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
+    <meta name="theme-color" content="#0f0f0f" />
+    <meta name="mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-status-bar-style" content="black" />
+    <meta name="apple-mobile-web-app-title" content="Luma" />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <link rel="apple-touch-icon" href="/icons/apple-touch-icon.png" />
     <title>Luma</title>
   </head>
   <body>