Add Immersive Canvas toggle to fit image above the floating toolbar

[thetalkingdrum]

Summary

The floating action toolbar sits over the bottom of the canvas. On portrait-orientation frames the pill can land on the image itself rather than just below it — how much depends on screen size/resolution — which is distracting when judging composition.

Immersive Canvas (toolbar overflow ⋯ menu, checkable) controls this:

• On (default): unchanged — the image fills the full canvas and the toolbar overlaps it, same as today.
• Off: fit-to-window reserves space for the toolbar's height, so the image is never occluded by it.

Kept as an opt-in toggle rather than a default change, since maximizing screen usage by overlapping the toolbar is presumably a deliberate choice for some setups/screens too, not just an oversight — this just gives people who find it distracting a way out without taking that away from anyone else. Worth keeping in mind if a floating-toolbar rework is ever on the table.

The setting persists across restarts and has a bindable (currently unassigned) shortcut via the shortcut editor.

Test plan

• make all (3421 passed, 1 pre-existing unrelated test_overflow_bar.py failure — confirmed failing identically on main before this change)
• Manual: toggled on/off on portrait and landscape frames at a few window sizes — image is never covered by the toolbar when off, unchanged when on
• Setting persists across an app restart