UI-A4 — High refresh rate enablement and debug overlay

[ZhuchkaTriplesix]

Epic: #170
Depends on: 0.4.3 shipped (can run parallel with #173–#172)

Source: docs/motion-and-high-refresh.md §3

Goal

Unlock native display refresh rate (90/120/144 Hz) where the engine/OS caps at 60, and add diagnostics to prove real Hz on each platform.

Files

• lib/main.dart — call unlock after WidgetsFlutterBinding.ensureInitialized()
• lib/core/motion/display_refresh.dart (new wrapper around refresh_rate package)
• pubspec.yaml — add refresh_rate dependency
• Optional: debug overlay gated by kDebugMode or hidden flag

Implementation

• Add refresh_rate package; thin DisplayRefreshService with enable(), queryHz(), disable().
• Call enable() / preferMax() once in main() on desktop.
• macOS 14+: primary unlock target (ProMotion/high-Hz).
• Windows / Linux: query and log active Hz; verify engine follows monitor (no unlock API needed per research).
• Debug-only FPS/Hz overlay (e.g. refresh_rate showHz()) behind a compile-time or runtime flag — not in release UI.
• Document measured Hz per OS in issue comment or A6 doc.

Acceptance Criteria

• App reports actual Hz ≥ monitor rate on macOS 14+ test machine (or logs warning if capped).
• Windows/Linux: Hz query logged at startup; manual DevTools confirms frame budget at target Hz.
• No regression in startup time or crash on platforms without high-Hz.
• flutter analyze clean; tests green.

References

• flutter/flutter#160952
• https://pub.dev/packages/refresh_rate

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