Remove stones as a weight unit, keeping only pounds and kilograms

[Copilot]

This PR removes support for stones as a weight unit from the Dropped cycling app, focusing only on imperial (pounds) and metric (kilograms) units as requested in the issue.

Changes Made

Core Model Updates

• WeightUnit enum: Removed .stones = "st" case from the enum
• Conversion logic: Removed stones conversion logic from the convert() function
• Migration: Added automatic migration for existing users with stones to convert them to pounds

UI Simplification

• OnboardingView: Removed the stones picker UI that appeared in imperial mode
• SettingsView: Removed stones option from the weight unit picker
• OnboardingViewModel: Removed showStonesOption property and related logic

Test Updates

• UserDataTests: Removed stones conversion tests and added migration verification
• OnboardingViewModelTests: Updated unit selection tests to focus on pounds ↔ kilograms conversion
• New tests: Added verification that only 2 weight units exist (pounds and kilograms)

Migration Strategy

Existing users who had stones selected will be automatically migrated to pounds when their data is loaded. The weight value remains stored in kilograms internally for consistency, but the display unit preference is changed from "st" to "lb".

Before/After Comparison

Before: 3 weight units (pounds, kilograms, stones)

enum WeightUnit: String, CaseIterable {
    case pounds = "lb"
    case kilograms = "kg" 
    case stones = "st"  // ❌ Removed
}

After: 2 weight units (pounds, kilograms)

enum WeightUnit: String, CaseIterable {
    case pounds = "lb"
    case kilograms = "kg"
}

The UI now presents a clean imperial/metric toggle without the additional complexity of stones selection.

Fixes #11.

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[Copilot]

Pull Request Overview

This PR removes support for stones as a weight unit, streamlines the UI, and adds migration for existing stone-based users to pounds.

• Removed .stones cases and conversion logic from models
• Simplified onboarding and settings views to only handle pounds and kilograms
• Introduced a migration path in UserDataManager and updated tests accordingly

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Dropped/Models/UserData.swift	Removed stones enum cases and conversion logic; added migration logic
Dropped/Views/SettingsView.swift	Removed stones picker UI
Dropped/Views/OnboardingView.swift	Removed stones option in onboarding
Dropped/ViewModels/OnboardingViewModel.swift	Removed showStonesOption and related logic
DroppedTests/UserDataTests.swift	Removed stones tests; added migration and allCases verification
DroppedTests/OnboardingViewModelTests.swift	Updated tests to reflect pounds↔kilograms flows

[Copilot]

The migration logic updates in-memory userData but does not persist these changes to storage. Consider calling saveUserData(migratedData) within this block to ensure the migration is saved.

Suggested change

	self.userData = migratedData
	saveUserData(migratedData)

[Copilot]

[nitpick] Avoid using the hardcoded literal "st". Define a legacy constant or extension to represent the old stones unit, which improves readability and reduces magic strings.

Suggested change

	if userData.weightUnit == "st" {
	if userData.weightUnit == WeightUnit.stones.rawValue {