fix: use dummy backend when no output device is availible

[liquidhelium]

fixes #605 .
Testing steps: disable all your audio devices in your system settings, then run phira.
Things to be discussed: Should the translation string be put into a seprate file, rather than scene.ftl?

[Copilot]

Pull request overview

This PR adds a fallback mechanism to gracefully handle scenarios where no audio output device is available. When audio backend initialization fails, the application now displays an error message and falls back to a dummy backend that silently ignores all audio operations, allowing the application to continue running without sound.

Key changes:

• Added error handling in build_audio() to catch backend initialization failures and use a dummy backend as fallback
• Implemented a minimal DummyBackend that satisfies the Backend trait interface without performing any actual audio operations
• Added localized error messages in English and Chinese to inform users when running in silent mode

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
prpr/src/ui.rs	Added error handling for audio backend initialization, implemented DummyBackend struct, and added l10n macro import for translation strings
prpr/locales/en-US/scene.ftl	Added English error message for audio backend initialization failure
prpr/locales/zh-CN/scene.ftl	Added Chinese error message for audio backend initialization failure

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