feat: PWA update strategy with hashed assets and cache control

Move frontend sources to frontend/, build content-hashed assets at deploy,
network-first SW for shell/JS, immutable cache for /assets/, version.json
update banner, and Cache-Control middleware. Docs in docs/PWA-STRATEGY.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/manifest.json

@@ -0,0 +1,48 @@
+{
+  "name": "TakeYourMeds",
+  "short_name": "Medis",
+  "description": "Medikamenten-Erinnerung mit Sarkasmus und Dopamin",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#1a1a2e",
+  "theme_color": "#F12F12",
+  "orientation": "portrait-primary",
+  "icons": [
+    {
+      "src": "/icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ],
+  "shortcuts": [
+    {
+      "name": "Genommen (Morgens)",
+      "short_name": "Morgens ✓",
+      "url": "/?action=take&slot=morning",
+      "icons": [
+        {
+          "src": "/icon-192x192.png",
+          "sizes": "192x192"
+        }
+      ]
+    },
+    {
+      "name": "Genommen (Mittags)",
+      "short_name": "Mittags ✓",
+      "url": "/?action=take&slot=noon",
+      "icons": [
+        {
+          "src": "/icon-192x192.png",
+          "sizes": "192x192"
+        }
+      ]
+    }
+  ]
+}