Default transcribe shortcut (Ctrl+Space) conflicts with ibus input-method switch on Linux

Environment: Ubuntu, GNOME (Wayland), Handy 0.9.8.

The default transcribe binding is Ctrl+Space. On any system running ibus (the default input method framework on most Linux desktops, including stock Ubuntu/GNOME), Ctrl+Space is already bound globally as the input-method switch trigger:

$ gsettings get org.freedesktop.ibus.general.hotkey trigger
['Control+space', 'Zenkaku_Hankaku', 'Alt+Kanji', 'Alt+grave', 'Hangul', 'Alt+Release+Alt_R']

Registering Handy's default shortcut on Linux fails silently against this:

register_tauri_shortcut duplicate error: Shortcut 'ctrl+space' is already in use
resume_all_shortcuts: could not register 'transcribe': Shortcut 'ctrl+space' is already in use

There is no user-facing error for this, the shortcut simply never fires. A new user has no way to know the default binding is dead on their system without checking the log file manually.

Suggested fix: either pick a Linux default that does not collide with ibus (e.g. Ctrl+Alt+Space), or detect the registration failure and surface it in the UI instead of only logging it.