Fix shield icon rendering, move slot machine to a movable panel

The shield icon's dark/mushy rendering was caused by disabled mipmaps
on all 6 slot-machine icon textures (inherited from the flat lucide
icon convention, wrong for these gradient-heavy assets) - at small
display sizes this caused aliasing that hit the shield's fine detail
(RZ lettering, metallic bevels) hardest since the other icons are
bolder/simpler shapes. Enabling mipmaps/generate on all 6 fixes it;
confirmed by rendering the actual in-game presentation size before
and after, not just eyeballing the source file.

Also converts the slot machine from a full standalone screen (sidebar
nav entry, main.gd-managed) into a movable/draggable floating panel
opened from the question-player toolbar, mirroring the existing
calculator panel exactly: same top_level+custom title-bar-drag+
viewport-clamp mechanics, same open/close/toggle API, instantiated
once alongside the calculator and closed on session hide. Shrunk the
cabinet and restacked the rewards list below it (was beside it) to
fit a toolbox-sized floating panel instead of a full page, wrapped in
a ScrollContainer for any overflow.

Removed the sidebar entry and screens[] registration from main.gd and
the now-dead routing case from navigation_routes.gd.

scripts/ui/navigation_routes.gd

@@ -31,6 +31,4 @@ static func key_for_screen(screen_name: String, payload: Variant = null) -> Stri
 		key = "donations"
 	elif screen_name == "rz_store":
 		key = "rz_store"
-	elif screen_name == "slot_machine":
-		key = "slot_machine"
 	return key