Replaces ad-hoc TouchableOpacity+styled-Text pairs with a single
`<Button>` covering the four variants we actually use (primary,
secondary, ghost, destructive), with size (sm/md/lg), loading,
disabled, icon, iconPosition, and a style escape hatch.
Migrated files: AddMacSheet, AddWindowsSheet, PlanChangeSheet,
devices.tsx CTA, settings SubscriptionSheet CTA.
Skipped (kept as-is to avoid hostile overrides): auth flow buttons
(Google/Apple OAuth with custom SVGs), list-row Touchables, blocker
& mail components (separate sweep when those screens come up).
paddingVertical default 12 (md) — matches the slimmer-buttons direction
we landed on in the devices-page redesign.
e4ac3ae51c