Feature request: 5% snap toggle for battery limit slider

**Is your feature request related to a problem? Please describe.**
The battery charge limit slider in AlDente snaps to 1% increments. I think it's my OCD talking, but I always
set mine to round 5% values (75%, 80%, 85%, 90%), so I have to spend 2-3
extra seconds every time fine-tuning to land on the right spot.

Not really an issue, that'd just be a nice to have.

**Describe the solution you'd like**
A toggle in Preferences to snap the slider to 5% increments instead of 1%.

Describe alternatives you've considered
- A modifier key (hold Shift) to temporarily enable 5% snapping while dragging.

**Describe alternatives you've considered**
None

**Additional context**
None

Start at the Preferences UI and the battery charge limit slider implementation, then trace how the current limit is stored and updated. Add a preference-controlled 5% snapping mode while preserving 1% snapping by default, and verify that dragging the slider produces the expected values and that the setting persists.

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