fix: recover from mpv initialization failures in Store builds

[StephanOrgiazzi]

Summary

• retry libmpv initialization with a conservative gpu renderer and software decoding when the preferred gpu-next profile fails
• preserve and display native Tauri/libmpv initialization errors instead of replacing them with the generic media-open failure
• disable file and URL opening until the playback engine is ready
• add coverage for serialized backend errors and both initialization profiles
• run unit tests in the Microsoft Store packaging workflow

Why

Microsoft Store certification reported Failed to play media file on a Dell Latitude 5410. mpv's loadfile command returns before the file is actually opened, so that message points to an unavailable mpv instance rather than an unsupported codec. The existing lifecycle caught the startup failure while leaving the open controls interactive, which masked the original cause.

Validation

• unit tests passed on Windows
• production Tauri build passed
• Store MSIX packaging passed
• Windows App Certification Kit passed