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/.metadata

@@ -15,7 +15,7 @@ migration:
     - platform: root
       create_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
       base_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
-    - platform: macos
+    - platform: web
       create_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42
       base_revision: 559ffa3f75e7402d65a8def9c28389a9b2e6fe42