Make SuiviCPT an installable PWA

Adds what iOS/Android need to treat the site as an installable app
from Safari's "Add to Home Screen": a manifest.json, apple-touch-icon
set generated from the existing logo mark (scripts/generate-icons.mjs,
sharp as a dev dependency), and the apple-mobile-web-app-* meta tags
iOS Safari requires since it ignores manifest.json entirely.

The service worker is deliberately minimal — this app shows live
financial data, so navigations always hit the network first. It only
caches enough to serve offline.html when there's no connection, not a
full offline-first app shell.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/manifest.json

@@ -0,0 +1,32 @@
+{
+    "name": "SuiviCPT",
+    "short_name": "SuiviCPT",
+    "description": "Suivi budgétaire et transactions pour le foyer",
+    "start_url": "/dashboard",
+    "scope": "/",
+    "display": "standalone",
+    "orientation": "portrait",
+    "background_color": "#f5ead8",
+    "theme_color": "#c67139",
+    "lang": "fr",
+    "icons": [
+        {
+            "src": "/icons/icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png",
+            "purpose": "any"
+        },
+        {
+            "src": "/icons/icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "any"
+        },
+        {
+            "src": "/icons/icon-maskable-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "maskable"
+        }
+    ]
+}