feat(pwa): web manifest, icons, read-through entry cache

- Add manifest.webmanifest, generated 192/512 icons, theme-color, apple-touch-icon
- Cache entries in localStorage per user; hydrate on load before network refresh
- Single info toast when offline on cached data; cache cleared on logout
- Fix inaccurate local-first meta description

public/manifest.webmanifest

@@ -0,0 +1,22 @@
+{
+  "name": "Red Bull Intake Tracker",
+  "short_name": "Intake",
+  "description": "Track Red Bull and caffeine intake — cans, spending, flavours, and trends — synced across devices.",
+  "start_url": ".",
+  "display": "standalone",
+  "background_color": "#eef3fb",
+  "theme_color": "#2563c7",
+  "icons": [
+    {
+      "src": "icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}