Replace ChartsPackage with native Swift Charts

- Rewrite HeaderStatsView using SwiftUI Charts framework
- Delete unused GraphView.swift (only used in previews)
- Remove unused `import Charts` from DayView.swift
- Remove ChartsPackage SPM dependency from project
- Native Swift Charts is simpler and has no external dependencies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Shared/Views/DayView/DayView.swift

@@ -7,7 +7,6 @@
 
 import SwiftUI
 import SwiftData
-import Charts
 import TipKit
 
 struct DayViewConstants {