rip out media_kit, use better_player and audioplayers

This also let us rip out cocoapods stuff.

Needs testing on Windows, MacOS, and ideally, iOS.

.github/workflows/windows.yml

@@ -45,6 +45,17 @@ jobs:
                   flutter pub run build_runner build
                   flutter build windows --dart-define=GIT_COMMIT=${{ github.sha }}
 
+            # better_player plays videos with libmpv, but doesn't bundle it
+            - name: Download libmpv
+              shell: powershell
+              run: |
+                  $archive = "$env:RUNNER_TEMP\mpv.7z"
+                  Invoke-WebRequest "https://github.com/shinchiro/mpv-winbuild-cmake/releases/download/20261021/mpv-dev-x86_64-20261021-git-e471496c56.7z" -OutFile $archive
+                  if ((Get-FileHash $archive -Algorithm SHA256).Hash -ne "efc719eff3dc5954b4a5efc800632808bcec01cc9034940dd14e5eb7fe9ea633") {
+                      throw "libmpv checksum mismatch"
+                  }
+                  7z e $archive libmpv-2.dll -obuild/windows/x64/runner/Release
+
             - name: Copy MinGW runtime DLLs
               shell: msys2 {0}
               run: |