Make the slot machine its own sidebar entry; rework its visuals

Moves Slot Machine into the sidebar's account section (alongside RZ
Shop/Settings/Log out) instead of being reachable only from the shop -
removes the shop's promo banner and the screen's "Back to Shop" link
accordingly.

Also closes a chunk of the visual gap against the mockup: a proper
cabinet-style panel (arched top corners, glowing border), a light
"screen" panel holding a real 3x3 reel grid instead of 3 single icons,
a decorative lever, an "RZ" marquee, and the cost/spin/auto-spin
controls each in their own bordered box. Added two new vendored icons
(gem, flask-conical) for reel variety and reward-list icons.

scripts/ui/navigation_routes.gd

@@ -32,5 +32,5 @@ static func key_for_screen(screen_name: String, payload: Variant = null) -> Stri
 	elif screen_name == "rz_store":
 		key = "rz_store"
 	elif screen_name == "slot_machine":
-		key = "rz_store"
+		key = "slot_machine"
 	return key