fix(a11y): Make mouse-wheel zoom also configurable with accessibility_zoom.increment

[luukvanderduim]

Hooked mouse-scroll zoom up to the accessibility_zoom.increment configuration instead of using a fixed 5% step.

Magnification through keyboard Super + = / - already has the step size hooked up to the configurable accessibility_zoom.increment. This commit unifies the behaviour between mouse-wheen and keyboard controlled zoom.

This also makes one mouse wheel click approximately the same size as one keyboard step size. Closing the gap of perceived differences between both modes of operation.

LLM disclosure:
Gemini Flash 3.5 was helpful in finding where the problem was located and suggested the initial fix.
That said, each line introduced is simple, well tested and well understood.

Tested on: x86_64 Xubuntu 26.04 with Cosmic Epoch 1.4 and a patched cosmic-comp.

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[rillbert]

Tested the rebased branch on Pop!_OS 24.04, ThinkPad with AMD integrated graphics driving an external monitor. Built with cargo build --profile fastdebug and ran it as my normal session.

Works well for me. At the default scroll speed with a 50% zoom increment I get a zoom doubling in about 3 detents, for the mousewheel which is comfortably usable. Before this I had to push the mouse scroll speed to ~65 to get fast enough zoom to be usable, which made scrolling in Firefox far too coarse. I haven't seen any regression in normal scrolling.

[luukvanderduim]

Works well for me. At the default scroll speed with a 50% zoom increment I get a zoom doubling in about 3 detents, for the mousewheel which is comfortably usable. Before this I had to push the mouse scroll speed to ~65 to get fast enough zoom to be usable, which made scrolling in Firefox far too coarse. I haven't seen any regression in normal scrolling.

It took me a while before I understood you had found a workaround by increasing mouse scroll-speed, which had unwanted side-effects.

Glad to hear this works for you too.