fix(editor): wheel scrolling and zoom in Firefox, Shift+wheel and small zoom steps

[AndyScherzinger]

Fixes #93

In Firefox the mouse wheel did nothing on the mind map canvas: scrolling didn't move the canvas, vertically or horizontally, and Ctrl+wheel didn't zoom. Both work in Chrome. In every browser, Shift+wheel didn't scroll horizontally, and touchpad pinches and small wheel steps didn't zoom.

The vendored KityMinder pans on the non-standard mousewheel event and zooms on it when Ctrl or Cmd is held:

• Firefox: it never fires mousewheel, and kity doesn't even listen for it there. kity only attaches a DOM listener when the element has an on<type> property ("onmousewheel" in node), which Firefox elements don't have. That also rules out KityMinder's own DOMMouseScroll fallback, so Firefox got no wheel handling at all.
• Shift: the pan handler ignores the event while Shift is held.
• Zoom: it only zooms when a single event reports more than 100. A mouse notch in Chrome reports 120, but touchpad pinches and the small pixel steps of macOS never get there.

src/viewer.js, the editor iframe's script, now handles the wheel before the editor does. The vendored files are unchanged.

• Firefox: elements get an onmousewheel property, so kity binds its listener, and every standard wheel event is re-dispatched as mousewheel with the deltas Chrome reports: 120 per notch of three lines, and 1.2 per pixel.
• Shift+wheel, all browsers: re-dispatched without Shift and with the delta moved to the horizontal axis, so it scrolls the canvas sideways.
• Ctrl/Cmd+wheel, all browsers: re-dispatched with at least 120, keeping its direction. The editor waits for 100 ms of quiet before it zooms, so a pinch or a fast series of steps still zooms once per gesture.

In Chrome the native mousewheel passes through untouched in all other cases. When the editor handles an event, the original is cancelled, so the page itself doesn't scroll or zoom too.

Tested on Nextcloud 32, 35 and 36, in Chromium and Firefox, by reading the editor's canvas position and zoom. The results are the same in both browsers and on all three versions:

Input	Result
Wheel down / right	Canvas scrolls 48 px vertically / horizontally (Firefox before: nothing)
Shift+wheel	Canvas scrolls 48 px horizontally (before: nothing in either browser)
Ctrl+wheel, one notch	Zoom 100 → 120 → 100 % (Firefox before: nothing)
Ctrl+wheel, 5 px step	Zoom 100 → 120 → 100 % (before: nothing)
Cmd+wheel, as on macOS	Zoom 100 → 120 %
12 small Ctrl steps in a row, like a pinch	One zoom step (120 → 100 %)
One mouse notch in Firefox line mode (simulated)	48 px, same as Chrome

macOS itself wasn't available for testing; the Cmd and small-step cases cover what macOS sends. Lint, Vitest and the build pass. Backport: #403.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI (N/A)