Release v1.2.0

Fixes

• Fix key-press debounce race condition, volume element detection, SystemEventMonitor cleanup, device menu unnecessary rebuilds, and device switch error alert title
• Fix SystemEventMonitor data race: global monitor callback dispatched to MainActor
• Fix SystemEventMonitor deinit: synchronize monitor removal on main thread
• Eliminate all MainActor.assumeIsolated: use Task { @MainActor in } for completion handlers and animation callbacks

Improvements

• Upgrade to Swift 6 language mode with strict concurrency checking
• Refactoring: consolidate ThreadSafeProperty, remove redundant code paths and unused abstractions, improve naming
• Concurrency: enable warnings-as-errors, mark pure types Sendable/nonisolated, use OSAllocatedUnfairLock(initialState:) for compiler-checked locking
• Tests: add SPM test target, skip hardware-dependent tests, clean up mocks, add dispatchPrecondition and assertForOverFulfill
• Formatting and documentation