chore(deps): drop unused cupertino_icons + tslib

Both verified unused at the source/config level (Fable #461 +
audit note #460):

- cupertino_icons: zero `CupertinoIcons` / cupertino imports in
  flutter_client/lib/. Pure `flutter create` template residue;
  the design system mandates Lucide.
- tslib: web/tsconfig.json does not set `importHelpers: true`,
  so TypeScript inlines helpers per-file. Declared peer with no
  runtime consumer.

`npm uninstall tslib --save-dev` updated package-lock.json
surgically (8 lines removed for the tslib entry only). No other
deps disturbed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/pubspec.yaml

@@ -33,7 +33,6 @@ dependencies:
   # until either lib bumps win32 to 6.x.
   package_info_plus: ^8.3.1
   pub_semver: ^2.1.4
-  cupertino_icons: ^1.0.8
   path_provider: ^2.1.5
   drift: ^2.18.0
   drift_flutter: ^0.2.0