docs: Add Flutter app design specification

Add the full design specification for the MyIce Flutter mobile app.
The app connects to the existing FastAPI backend and replicates
the webapp functionality with OIDC auth, schedule browsing,
and event detail views.

.pre-commit-config.yaml

@@ -40,7 +40,7 @@ repos:
     rev: v0.13.0
     hooks:
       - id: markdownlint
-        exclude: "^.github|(^docs/_sidebar\\.md$)"
+        exclude: "^.github|_sidebar\\.md$|superpowers/specs/.*\\.md$"
 
   - repo: https://github.com/shellcheck-py/shellcheck-py
     rev: v0.11.0.1