feat(desktop): add presentation mode and its effects

Presentation mode sets DND, asserts a Wayland idle inhibitor and pauses
breaktimer. The effects hang off the mode property rather than the setter,
so a mode set with statusctl while the drawer is closed asserts them too.

DND has two writers once presentation mode exists, so turning presentation
off restores the value DND had before rather than clearing it, or an
afternoon of hand-set DND would vanish when a talk ends. That prior value
lives in the singleton, not in a file: it means nothing once presentation
mode is off, and presentation mode does not survive a reboot.

breaktimer owns its own state file and is driven only through its verbs.
Two writers on that file would race with its daemon loop.

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