KEY-ZOOM: Cmd+= / Cmd+- / Cmd+0 zoom the photo, not egui's interface scale (#566)

egui's zoom_with_keyboard was left on, so at the end of every frame it took the
same keys as View ▸ Zoom In / Zoom Out / Zoom to Fit for its browser-style
interface zoom. On macOS the native menu bar got the keys first; on Windows and
Linux the whole UI scaled. theme::apply turns the option off for every context
(desktop, headless, tests).

Headless tests that pressed Cmd+Plus / Cmd+Minus to exercise the interface
zoom's viewport handling set the zoom factor directly; the CLI snapshot test
that did the same is dropped (the unit tests cover the mechanism). New
tests_zoom_keys.rs: the shortcuts step the photo zoom while the zoom factor
stays, and Ctrl+wheel over the photo zooms it in the Edit view.

docs/control-protocol.md

@@ -100,7 +100,8 @@ Headless screenshot dimensions must be finite and at least one logical point, wi
 positive scale. Rounded output must be at least one pixel per edge, at most 16,384 pixels
 per edge and 64 million pixels in total. `snapshot` and `ui.resize` reject requests outside
 these limits before layout or rasterization; supplied resize dimensions must be numbers.
-UI zoom changes text and control sizes while preserving the snapshot viewport's requested
+An interface zoom (egui's zoom factor; the keyboard never changes it — ⌘= / ⌘− / ⌘0 zoom the
+photo, issue #566) changes text and control sizes while preserving the snapshot viewport's requested
 physical pixel dimensions. Later `ui.resize` requests use current egui points; validation
 and rasterization both convert them to native viewport points before rounding the output
 pixels. A native edge below one point after zooming out is valid if it rounds to at least