a11y: magnifier: change zoom stepping to multiplicative, bound max, add presets

1. Change magnifier zoom stepping from additive to multiplicative,
   because zoom is a ratio, not a difference, and human perception is
   logarithmic (browser zoom steps, map zoom levels, microscope powers).

2. Add `pub const ZoomConfig::ZOOM_INCREMENT_PRESETS` with two
   additional presets: 10% (macOS, Cinnamon) and 75% (for users at
   70-90x zoom).

3. Bound the zoom level to a generous maximum, beyond what anyone is expected
   to need, to prevent runaway / unrecoverable levels (#1230).

Full background and rationale at #2615.

I used AI to assist with research and issue writeup. Code is 100% mine.

- Closes #2615.
- Fixes #1239.
- Partially addresses #1230 and #1237.

cosmic-comp-config/src/lib.rs

@@ -198,6 +198,10 @@ pub struct ZoomConfig {
     pub enable_mouse_zoom_shortcuts: bool,
 }
 
+impl ZoomConfig {
+    pub const ZOOM_INCREMENT_PRESETS: &[u32] = &[10, 25, 50, 75, 100, 150, 200];
+}
+
 impl Default for ZoomConfig {
     fn default() -> Self {
         ZoomConfig {