Add detailed statistics on iOS

[austin-smith]

What Changed

• Added an iOS-native Statistics sheet with live, current-session, and cumulative Transmission statistics.
• Added responsive layouts for torrent counts, speeds, metric values, Dynamic Type, and the no-data state.
• Made the shared statistics header open the platform-appropriate presentation: a sheet on iOS and the existing Statistics window on macOS.
• Preserved the header’s live ratio and transfer speeds in its VoiceOver value.
• Increased the macOS Statistics window’s ideal height so its full content is discoverable.
• Added shared ratio and active-duration formatting helpers with regression tests.

Why

iOS exposed the compact statistics header but had no way to view the detailed session statistics already available through TransmissionStore.sessionStats.

Closes #122.

UI Changes

• Tapping the statistics header presents a dismissible Statistics sheet on iOS.
• The iOS form adapts its count and metric rows to available width and accessibility text sizes.
• Clicking the statistics header opens the Statistics window on macOS without changing the existing form appearance.

Validation

• macOS test suite: 212 passed
• iOS generic-device Debug build with signing disabled
• SwiftLint
• git diff --check