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.

assets/slot_machine/lever.png.import

@@ -23,7 +23,7 @@ compress/rdo_quality_loss=0.0
 compress/hdr_compression=1
 compress/normal_map=0
 compress/channel_pack=0
-mipmaps/generate=false
+mipmaps/generate=true
 mipmaps/limit=-1
 roughness/mode=0
 roughness/src_normal=""