This commit introduces a centralized `SnackbarManager` to handle transient feedback across the application, decoupling ViewModels from specific UI implementations. It also consolidates global UI components and side-effect handlers (alerts, version checks, traceroute responses) into a shared `MeshtasticCommonAppSetup` to eliminate boilerplate across platform-specific entry points.

Specific changes include:

- **Core UI & Infrastructure:**
  - Introduced `SnackbarManager` in `core:ui` to manage snackbar events via a buffered `Channel`.
  - Created `MeshtasticSnackbarProvider` and `MeshtasticSnackbarHost` to render global snackbars at the root level.
  - Implemented `MeshtasticCommonAppSetup` to unify `SharedDialogs`, `FirmwareVersionCheck`, `AlertHost`, and `TracerouteAlertHandler`.
  - Added `navigateTopLevel` extension for `NavBackStack` to standardize top-level navigation logic.

- **Feature Node Refactoring:**
  - Replaced local `SnackbarHostState` and `NodeRequestEffect` with global `SnackbarManager` across all metrics screens (`HostMetrics`, `PaxMetrics`, `PowerMetrics`, `SignalMetrics`, `EnvironmentMetrics`, `DeviceMetrics`, `PositionLog`, etc.).
  - Updated `CommonNodeRequestActions` to inject `SnackbarManager` and trigger feedback directly.
  - Removed `effects` flow from `NodeDetailViewModel` and `MetricsViewModel`.

- **Platform-Specific Updates:**
  - **Android:** Updated `Main.kt` to use `MeshtasticCommonAppSetup` and `MeshtasticSnackbarProvider`. Refactored version check logic.
  - **Desktop:** Integrated `DesktopNotificationManager` to bridge repository notifications with the system tray/Compose `TrayState`.
  - **Desktop:** Added native macOS support, including `bundleID` configuration and monochrome SVG tray icons.
  - **Desktop:** Updated `DesktopMainScreen.kt` to use the new common setup and snackbar provider.

- **Documentation & Maintenance:**
  - Updated `AGENTS.md` and `GEMINI.md` to reflect new guidelines for using `TracerouteAlertHandler` and `FirmwareVersionCheck`.
  - Added unit tests for `SnackbarManager`.
  - Updated various tests to align with the removal of `NodeRequestEffect`.

Signed-off-by: James Rich <[email protected]>