Add an RZ Coin slot machine

Adds a spend-coins-for-a-random-reward mini-game reachable from the
shop. Backend mirrors the store's existing atomic/idempotent purchase
pattern (SpinAsync in CreditStore.cs) with a server-authoritative,
weighted reward roll and a rolling per-user spin-rate guard; odds are
served from GET /api/credits/spin-config so the client never hardcodes
numbers that could drift from the real payout table. The mockup's
"Streak Protector" reward was swapped for a real Exam Simulation Token
since streaks aren't currently a persisted, protectable concept.

Also fixes two latent bugs surfaced while building this:
- credits_service.gd's operation-id builder referenced a nonexistent
  AuthState.user_id (should be .username), throwing on every credit
  action.
- _post() silently no-op'd with no signal when signed out, which
  combined with a GDScript closure gotcha (lambdas capture outer
  locals by value, so reassigning inside one never reached the
  caller) meant a spin attempted while unauthenticated could hang
  forever instead of surfacing an error.

scripts/ui/rz_store_screen.gd

@@ -28,6 +28,10 @@ var balance_value_label: Label
 var coin_caption_label: Label
 var inventory_label: Label
 var tab_controls: Array = []
+var banner_card: PanelContainer
+var banner_icon_panel: PanelContainer
+var banner_heading: Label
+var banner_description: Label
 var section_heading: Label
 var section_subtitle: Label
 var products_grid: HFlowContainer
@@ -49,6 +53,7 @@ func _ready() -> void:
 	_build_header()
 	_build_summary_card()
 	_build_tabs()
+	_build_slot_machine_banner()
 
 	section_heading = Label.new()
 	section_heading.text = "Featured"
@@ -217,6 +222,63 @@ func _apply_tab_theme(dark: bool) -> void:
 		(entry["underline"] as ColorRect).color = Brand.ROYAL_BLUE if active else Color(0, 0, 0, 0)
 
 
+func _build_slot_machine_banner() -> void:
+	var dark := UIThemeService.is_dark_mode()
+	banner_card = PanelContainer.new()
+	banner_card.add_theme_stylebox_override("panel", Brand.content_card_box(dark, true, Brand.ROYAL_BLUE))
+	add_child(banner_card)
+
+	var margin := MarginContainer.new()
+	margin.add_theme_constant_override("margin_left", 16)
+	margin.add_theme_constant_override("margin_right", 16)
+	margin.add_theme_constant_override("margin_top", 12)
+	margin.add_theme_constant_override("margin_bottom", 12)
+	banner_card.add_child(margin)
+
+	var row := HBoxContainer.new()
+	row.add_theme_constant_override("separation", 14)
+	margin.add_child(row)
+
+	banner_icon_panel = PanelContainer.new()
+	banner_icon_panel.custom_minimum_size = Vector2(40, 40)
+	banner_icon_panel.size_flags_vertical = Control.SIZE_SHRINK_CENTER
+	banner_icon_panel.add_theme_stylebox_override("panel", Brand.icon_circle_box(dark, Brand.ROYAL_BLUE, 20))
+	var icon := TextureRect.new()
+	icon.texture = load("res://assets/icons/lucide/star.svg")
+	icon.custom_minimum_size = Vector2(20, 20)
+	icon.expand_mode = TextureRect.EXPAND_IGNORE_SIZE
+	icon.stretch_mode = TextureRect.STRETCH_KEEP_ASPECT_CENTERED
+	icon.modulate = Brand.ROYAL_BLUE
+	banner_icon_panel.add_child(icon)
+	row.add_child(banner_icon_panel)
+
+	var copy := VBoxContainer.new()
+	copy.size_flags_horizontal = Control.SIZE_EXPAND_FILL
+	copy.size_flags_vertical = Control.SIZE_SHRINK_CENTER
+	copy.add_theme_constant_override("separation", 2)
+	banner_heading = Label.new()
+	banner_heading.text = "RZ Coin Slot Machine"
+	banner_heading.add_theme_font_override("font", Brand.font)
+	banner_heading.add_theme_font_size_override("font_size", 15)
+	banner_heading.add_theme_color_override("font_color", Brand.text_color(dark))
+	copy.add_child(banner_heading)
+	banner_description = Label.new()
+	banner_description.text = "Spend RZ Coins for a chance at bigger rewards."
+	banner_description.add_theme_font_override("font", Brand.font)
+	banner_description.add_theme_font_size_override("font_size", 12)
+	banner_description.add_theme_color_override("font_color", Brand.muted_color(dark))
+	copy.add_child(banner_description)
+	row.add_child(copy)
+
+	var play_button := Button.new()
+	play_button.text = "Play Now"
+	play_button.custom_minimum_size = Vector2(96, 36)
+	play_button.add_theme_font_override("font", Brand.font)
+	_style_buy_button(play_button)
+	play_button.pressed.connect(func(): main.show_screen("slot_machine"))
+	row.add_child(play_button)
+
+
 func _build_footer() -> void:
 	var dark := UIThemeService.is_dark_mode()
 	footer_card = PanelContainer.new()
@@ -276,6 +338,10 @@ func _refresh_theme_controls() -> void:
 	coin_caption_label.add_theme_color_override("font_color", Brand.muted_color(dark))
 	inventory_label.add_theme_color_override("font_color", Brand.muted_color(dark))
 	_apply_tab_theme(dark)
+	banner_card.add_theme_stylebox_override("panel", Brand.content_card_box(dark, true, Brand.ROYAL_BLUE))
+	banner_icon_panel.add_theme_stylebox_override("panel", Brand.icon_circle_box(dark, Brand.ROYAL_BLUE, 20))
+	banner_heading.add_theme_color_override("font_color", Brand.text_color(dark))
+	banner_description.add_theme_color_override("font_color", Brand.muted_color(dark))
 	section_subtitle.add_theme_color_override("font_color", Brand.muted_color(dark))
 	footer_card.add_theme_stylebox_override("panel", Brand.content_card_box(dark, false))
 	footer_icon_panel.add_theme_stylebox_override("panel", Brand.icon_circle_box(dark, Brand.ROYAL_BLUE, 18))