feat: use ConfettiSwiftUI package instead of custom confetti

- import ConfettiSwiftUI
- .confettiCannon modifier on MainTabView
- 80 particles, circles/crosses/rectangles, 7 colors
- Trigger: confettiTrigger += 1
- Removed all custom confetti code (ConfettiView, PhysicsModifier, etc.)

Requires: Add package in Xcode → https://github.com/simibac/ConfettiSwiftUI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/.stignore

@@ -0,0 +1,13 @@
+// Xcode user data — changes every build, ignore
+Platform.xcodeproj/xcuserdata
+Platform.xcodeproj/project.xcworkspace/xcuserdata
+Platform.xcodeproj/project.xcworkspace/xcshareddata
+
+// Build artifacts
+DerivedData
+build
+.build
+
+// macOS files
+.DS_Store
+*.xcuserstate