add desktop notifications for pipeline errors and typing failures

[zugaldia]

Summary

• Adds PortalsClient.showNotification() wrapping the XDG Desktop Notification portal via Stargate SDK
• Notifies the user when a pipeline stage fails (recording, transcription, or text processing) with a stage-specific message
• Notifies the user when text-to-keysym conversion fails, so typing errors are no longer silent

Test plan

• Trigger a pipeline error (e.g. disable the ASR model) and verify a desktop notification appears with the correct stage label
• Verify the notification title defaults to "Speed of Sound"
• Verify normal successful runs produce no notifications