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/main.gd

@@ -445,6 +445,7 @@ 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)
 
@@ -489,6 +490,7 @@ 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()