Implement global hotkey registration for Windows media key detection

[Copilot]

Windows media key detection only worked when SwiftControl was focused, preventing BLE HID controllers from functioning in the background.

Changes

Native Plugin (media_key_detector_windows_plugin.cpp)

• Replaced focus-dependent keyboard handler with RegisterHotKey API for VK_MEDIA_PLAY_PAUSE, VK_MEDIA_NEXT_TRACK, VK_MEDIA_PREV_TRACK
• Added event channel + window message handler for WM_HOTKEY events
• Implemented lifecycle management: register on setIsPlaying(true), unregister on setIsPlaying(false) or destruction
• All-or-nothing registration with cleanup on partial failure

Dart Plugin (media_key_detector_windows.dart)

• Switched from keyboard handler to event channel listener (consistent with iOS/macOS)
• Wire setIsPlaying to native method for hotkey control

Implementation

// Hotkeys registered globally with MOD_NOREPEAT
RegisterHotKey(hwnd, HOTKEY_PLAY_PAUSE, MOD_NOREPEAT, VK_MEDIA_PLAY_PAUSE);

// WM_HOTKEY messages forwarded to Dart via event channel
std::optional<LRESULT> HandleWindowProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam) {
  if (message == WM_HOTKEY && event_sink_) {
    event_sink_->Success(EncodableValue(key_index));
    return 0;
  }
  return std::nullopt;
}

Media keys now work system-wide regardless of application focus.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ensure BLE HID media controllers work with Windows</issue_title>
<issue_description>> I also wanted to ask if the Windows version now includes BLE HID support in version 3.4.0, or is there still only classic BT?

I couldn't connect my controller there in the application.

And one last thing: the web version redirects to the URL:
https://jonasbark.github.io/swiftcontrol/#/device
and a blank gray screen appears (tested in Edge and Chrome)

Originally posted by @TomasT342 in #110</issue_description>

<agent_instructions>the Windows implementation of the media_key_detector plugin (media_key_detector_windows_plugin.cpp) does not work globally when SwiftControl isn't the focused application. Implement a way to globally register for the media hotkeys</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Ensure BLE HID media controllers work with Windows #176

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