feat: add dynamic overview zoom controls

[barrulus]

Summary

• Add zoom-presets config option for overview
• Add overview-zoom-cycle, overview-zoom-in, overview-zoom-out actions
• Add overview-zoom animation config
• Zoom changes are animated smoothly
• Zoom resets to config default when overview closes

This allows users to dynamically adjust the overview zoom level to see more or fewer workspaces at once.

Design alignment

This implementation follows niri's design principles:

• Animated transitions: Zoom changes animate smoothly using a configurable spring animation, consistent with other niri animations.
• No performance impact when unused: If zoom-presets is not configured, zoom actions are no-ops. The runtime zoom state is minimal (one f64 per monitor).
• Mindful of invisible state: The zoom level resets to the config default when closing the overview, so reopening it always starts from a predictable state. Users won't be surprised by a forgotten zoom level from a previous overview session.

Example config

overview {
    zoom 0.5
    zoom-presets 0.5 0.25 0.1
}

animations {
    overview-zoom {
        spring damping-ratio=1.0 stiffness=800 epsilon=0.0001
    }
}

binds {
    Mod+Z { overview-zoom-cycle; }
    Ctrl+WheelScrollUp cooldown-ms=150 { overview-zoom-in; }
    Ctrl+WheelScrollDown cooldown-ms=150 { overview-zoom-out; }
}

Test plan

• Enter overview → should use config zoom value
• Press zoom cycle key → should animate through presets
• Ctrl+Scroll → should animate to next/prev preset
• Close overview → zoom should reset to config default
• Re-enter overview → should start at config default again
• No presets configured → zoom actions should no-op

Closes: discussion #3047

[youngcoder45]

Nice i really loved your idea zoomable overview hope this gets accepted