chore(web): enable Flutter web target

Adds the platform scaffolding flutter create produces for web
(web/index.html, manifest.json, favicon, icons). flutter build
web --release compiles the existing codebase to JS in ~16 s.
.metadata records the platform now lists web.

What works on web today:

  - Mock-mode UI (the same five fixture norms, sorted,
    inline-text-card, browser-open via launchUrl which becomes
    'open in new tab' in the browser).
  - All static screens (Heatmap, Normen, Methodik, Recht).

What does NOT yet work on web:

  - Hub-mode gRPC. chain_client_sdk_dart uses package:grpc,
    which in a browser needs the gRPC-Web wire format
    (text/grpc-web+proto over fetch). The Ch∆In Hub speaks
    gRPC-Web on the same port per docs/architecture/
    protocol-surfaces.md, but the client SDK needs a small
    web-channel branch to actually use it. Phase 1 task.

Why this matters: a single URL the AG can click is a different
political artefact than a downloaded .app. Web + macOS share
the same Recl∆Im codebase; nobody has to choose.

Signed-off-by: flemming-it <sf@flemming.it>

app/web/manifest.json

@@ -0,0 +1,35 @@
+{
+    "name": "reclaim_app",
+    "short_name": "reclaim_app",
+    "start_url": ".",
+    "display": "standalone",
+    "background_color": "#0175C2",
+    "theme_color": "#0175C2",
+    "description": "A new Flutter project.",
+    "orientation": "portrait-primary",
+    "prefer_related_applications": false,
+    "icons": [
+        {
+            "src": "icons/Icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-maskable-192.png",
+            "sizes": "192x192",
+            "type": "image/png",
+            "purpose": "maskable"
+        },
+        {
+            "src": "icons/Icon-maskable-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "maskable"
+        }
+    ]
+}