feat(pwa): add manifest shortcuts and a 192 maskable icon

Add three home-screen shortcuts (Add measurement, Today, Medications)
deep-linking into the app, each tagged `source=shortcut`. Add a 192×192
maskable icon alongside the existing 512 so adaptive launchers mask the
right size instead of downscaling the 512 or clipping the un-masked 192
"any" icon on circular masks.

No screenshots: the repo ships no real screenshot assets and fabricating
them is worse than the richer-install-dialog gain.

public/manifest.json

@@ -22,11 +22,37 @@
       "type": "image/png",
       "purpose": "any"
     },
+    {
+      "src": "/logo-maskable-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
     {
       "src": "/logo-maskable-512.png",
       "sizes": "512x512",
       "type": "image/png",
       "purpose": "maskable"
     }
+  ],
+  "shortcuts": [
+    {
+      "name": "Add measurement",
+      "short_name": "Add",
+      "url": "/measurements?add=WEIGHT&source=shortcut",
+      "icons": [{ "src": "/logo-192.png", "sizes": "192x192", "type": "image/png" }]
+    },
+    {
+      "name": "Today",
+      "short_name": "Today",
+      "url": "/?source=shortcut",
+      "icons": [{ "src": "/logo-192.png", "sizes": "192x192", "type": "image/png" }]
+    },
+    {
+      "name": "Medications",
+      "short_name": "Meds",
+      "url": "/medications?source=shortcut",
+      "icons": [{ "src": "/logo-192.png", "sizes": "192x192", "type": "image/png" }]
+    }
   ]
 }