Make the app installable (PWA metadata)

- manifest.webmanifest with name, standalone display, theme color
- PNG icons 192/512 + maskable variant + apple-touch-icon (from the
  Poké Ball favicon)
- minimal pass-through service worker (installability requirement)
- apple/mobile web app meta tags

public/manifest.webmanifest

@@ -0,0 +1,30 @@
+{
+    "name": "PokeShare",
+    "short_name": "PokeShare",
+    "description": "Gérez et partagez votre collection de cartes Pokémon TCG.",
+    "start_url": "/",
+    "scope": "/",
+    "display": "standalone",
+    "background_color": "#0a0a0a",
+    "theme_color": "#ee1515",
+    "icons": [
+        {
+            "src": "/pwa-192.png",
+            "sizes": "192x192",
+            "type": "image/png",
+            "purpose": "any"
+        },
+        {
+            "src": "/pwa-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "any"
+        },
+        {
+            "src": "/pwa-maskable-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "maskable"
+        }
+    ]
+}

public/sw.js

@@ -0,0 +1,16 @@
+/**
+ * Minimal service worker — network pass-through.
+ * Its only job is to make the app installable (browsers require a service
+ * worker to offer "Install app"); no offline caching for now.
+ */
+self.addEventListener("install", (event) => {
+    self.skipWaiting();
+});
+
+self.addEventListener("activate", (event) => {
+    event.waitUntil(clients.claim());
+});
+
+self.addEventListener("fetch", (event) => {
+    event.respondWith(fetch(event.request));
+});