Add Sticky Zoom to keep zoom level across image switches

[thetalkingdrum]

Summary

Switching to a different frame always resets the canvas to fit-to-window. That's normally the right first look, but sometimes a smaller-than-fit image reads its composition better — easier to judge as a whole rather than blown up — especially on a larger screen. Previously you'd zoom out to that size, then have to redo it on every next frame.

Sticky Zoom (toolbar overflow ⋯ menu, canvas right-click menu, and a bindable shortcut) keeps the current zoom level instead of resetting it when you move to another image. Off by default, so nothing changes unless you opt in, and persists across restarts like Immersive Canvas.

This is a fairly common convention in other image-viewing/editing apps, so it should feel familiar rather than novel.

Test plan

• make all: 3476 passed, 1 pre-existing unrelated test_overflow_bar.py failure (confirmed failing identically on main before this branch)
• Unit tests: set_sticky_zoom persistence (tests/test_desktop_session.py), navigation wiring — zoom resets by default, preserved when Sticky Zoom is on (tests/test_controller.py)
• Manual: zoomed out, switched frames repeatedly with Sticky Zoom on and off, confirmed it sticks/resets as expected across a session