feat: add dynamic overview zoom controls

[barrulus]

Summary

• Add zoom-presets, min-zoom, max-zoom, zoom-step config options for overview
• Add overview-zoom-cycle, overview-zoom-in, overview-zoom-out, overview-set-zoom actions
• Zoom changes apply instantly (no animation)
• 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:

• Actions apply immediately: Zoom changes take effect instantly with no animation, keeping the compositor responsive and predictable.
• No performance impact when unused: If zoom-presets is not configured, overview-zoom-cycle is a no-op. 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
    min-zoom 0.05
    max-zoom 0.75
    zoom-step 0.1
}

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 cycle through presets instantly
• Ctrl+Scroll → should adjust zoom within min/max bounds
• Close overview → zoom should reset to config default
• Re-enter overview → should start at config default again
• No presets configured → overview-zoom-cycle should no-op

Closes: discussion #3047

This code has been created with LLM assistance