feat(pwa): installable manifest and icons

static/manifest.webmanifest

@@ -0,0 +1,27 @@
+{
+  "name": "Firefly III",
+  "short_name": "Firefly",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#3c8dbc",
+  "icons": [
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ],
+  "shortcuts": [
+    { "name": "New expense", "url": "/transactions/create/withdrawal" },
+    { "name": "Transactions", "url": "/transactions/all" }
+  ]
+}