feat(mobile): establish loading state and skeleton screen pattern

[GRACENOBLE]

Problem

There is no established pattern for loading states in the mobile app. Each screen will invent its own approach, leading to inconsistent UX.

Acceptance criteria

• Create a reusable LoadingScreen composable (full-screen centered spinner, follows Material3 theme)
• Create a reusable SkeletonBox composable (shimmer placeholder for list items and cards)
• Add an example of each in the home or settings screen
• Document the pattern in mobile/docs/compose-conventions.md
• Add instrumented tests for both composables

[GRACENOBLE]

Implemented and merged via PR #66.