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>

Feels.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -1,15 +1,6 @@
 {
-  "originHash" : "e45fbe19a7a507ebcf572c1b8f6c29186a9e9e533901421771ee77df0ed47fc9",
+  "originHash" : "86635982bfc3fdf16c7186cd33e77b4dc10a24ff9127aaee5f4f0a3676ae2966",
   "pins" : [
-    {
-      "identity" : "chartspackage",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/akatreyt/ChartsPackage",
-      "state" : {
-        "branch" : "master",
-        "revision" : "4eb26e270cba2a2230d9c6b25c2ad629bc215e15"
-      }
-    },
     {
       "identity" : "swift-algorithms",
       "kind" : "remoteSourceControl",