The agreed stack and the structure that organises it. Defaults are committed where possible; remaining choices are listed in Section 20 — Open Decisions.

5.1 — Mobile app

Concern	Choice	Notes
Framework	Flutter	Stable channel. Lock the SDK version in pubspec.yaml.
Language	Dart	Sound null safety on.
Platforms	iOS, Android	Tablet layouts in scope; not a launch priority.
Architecture	Feature‑first clean architecture	One folder per feature; shared concerns in core/.
Rendering	CustomPainter	Single painter per canvas surface — see Section 11.
State	Riverpod 2.x	Pin major version. Avoid mixing with Provider.
Routing	GoRouter	Declarative routes; deep linking enabled.
HTTP	Dio	Interceptors for auth, retries, logging.
Local DB	Drift	Chosen over Isar — see Section 20.
Secure storage	flutter_secure_storage	For auth tokens and receipt cache.
IAP	in_app_purchase	StoreKit 2 on iOS, Billing 6+ on Android.
Files / cache	path_provider	Bundle cache under app‑support directory.
Path parsing	path_drawing	Required to parse SVG path strings — see Section 06.
Compression	brotli (Dart)	Used to decompress region payloads.
Remote config	GrowthBook or equivalent	For paywall toggles and staged rollouts.

Suggested module structure

lib/
  app/
  core/
    network/
    storage/
    analytics/
    config/
  features/
    library/
    artwork_detail/
    download/
    canvas/
    palette/
    progress/
    auth/
    paywall/
    admin_preview/
    settings/

5.2 — Backend

Concern	Choice
Platform	InsForge
Database	PostgreSQL (managed via InsForge)
API layer	PostgREST‑style auto‑generated APIs + edge functions for write paths
Authentication	InsForge Auth (email + Apple + Google)
Storage	InsForge Storage (S3‑compatible) fronted by a CDN
Authorisation	Row‑level security policies — see Section 07

InsForge is responsible for user authentication, the catalogue, asset hosting, entitlement records, purchase validation, progress sync, the admin publishing workflow, and the signed‑URL gateway for premium asset access.

5.3 — Asset storage layout

Artwork assets are stored remotely as structured bundles, one folder per artwork:

artworks/
  <slug>/
    manifest.json
    regions.bin.br          # binary, brotli-compressed
    preview.webp
    thumbnail.webp
    line_art.svg            # optional, reference only
    version.txt

The app downloads bundles on demand and caches them under the app‑support directory. Locale variants of manifest.json are not stored alongside the bundle — translatable strings live in PostgreSQL (see Section 07).