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/main.gd

@@ -361,11 +361,6 @@ func _build_ui() -> void:
 	rz_store.main = self
 	screens_container.add_child(rz_store)
 	screens["rz_store"] = rz_store
-	var slot_machine := preload("res://scripts/ui/rz_slot_machine_screen.gd").new()
-	slot_machine.name = "RzSlotMachine"
-	slot_machine.main = self
-	screens_container.add_child(slot_machine)
-	screens["slot_machine"] = slot_machine
 
 	feedback_dialog = preload("res://scripts/ui/app_feedback_dialog.gd").new()
 	feedback_dialog.name = "AppFeedbackDialog"
@@ -445,7 +440,6 @@ func _populate_navigation(container: VBoxContainer, include_brand: bool) -> void
 	_add_nav_button(container, "donations", "Support RanaZe", "heart-pulse", true)
 	_add_nav_button(container, "performance", "Profile", "user-round", true)
 	_add_nav_button(container, "rz_store", "RZ Shop", "star", true)
-	_add_nav_button(container, "slot_machine", "Slot Machine", "gem", true)
 	_add_nav_button(container, "settings", "Settings", "settings", true)
 	_add_nav_button(container, "logout", "Log out", "log-out", true)
 
@@ -490,7 +484,6 @@ func _on_navigation_pressed(key: String) -> void:
 		"bookmarks": show_screen("bookmarks")
 		"donations": show_screen("donations")
 		"rz_store": show_screen("rz_store")
-		"slot_machine": show_screen("slot_machine")
 		"logout": _logout()