Fix: world cards render at ~1px wide once real data exists

Reported directly by the user after seeding a test world through the
browse-worlds screen: "screen is blank now". Two compounding layout
bugs, both real:

1. world_list.tscn's root VBoxContainer had no anchors at all, only
   size_flags -- but its parent (the scene's root node) is a plain
   Control, not a Container, so size_flags on a Control's direct child
   are meaningless there. Without anchors it never actually filled the
   screen, so it (and everything under it) collapsed toward zero size.

2. size_flags_horizontal/vertical used the raw integer 4 everywhere
   expand-fill was clearly intended (ScrollContainer, its WorldList
   child, HeaderHBox, TitleLabel) -- but 4 is Control.SIZE_SHRINK_CENTER
   in Godot 4, not SIZE_EXPAND_FILL (which is 3). ScrollContainer with
   SHRINK_CENTER takes only its own minimum size instead of expanding,
   cascading a ~1px width down through every card and every label.
   Confirmed the actual enum values by asking Godot directly rather
   than assuming.

Both are exactly the kind of bug a headless smoke test checking only
"did cards get added to the tree" can't catch -- child_count was
correctly non-zero, nothing crashed, the content was just rendered
invisibly thin. Verified the fix is real by deliberately reintroducing
bug #2 and confirming the test now correctly fails (card width 25px,
below the 200px sanity threshold) before restoring it.

Also replaced the card background: it was a ColorRect manually moved to
child index 0 of a VBoxContainer, which doesn't work as an overlay --
a VBoxContainer lays out ALL its children in sequence, so the "moved to
front" ColorRect rendered as an empty dark bar ABOVE the text, not
behind it. Now a proper PanelContainer with a StyleBoxFlat background,
the idiomatic way to get a colored panel behind auto-laid-out content
in Godot.

smoke_test_world_list.gd strengthened to actually catch this class of
bug going forward: seeds a real world via POST /api/worlds/ first (the
checkpoint runner's fresh DB meant the populated-card path never ran in
CI at all before this, only empty-list did), and asserts each card's
real computed width, not just that cards exist.

Full checkpoint suite re-verified clean on this branch: migrations,
68/68 pytest, all 6 Godot smoke tests, both platform exports.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/world_list.gd

@@ -88,10 +88,26 @@ func _refresh_worlds() -> void:
 
 ## Create a visual card for a single world entry.
 func _create_world_card(world_dict: Dictionary) -> Control:
+	# PanelContainer for the background, not a ColorRect moved to child index
+	# 0 of a VBoxContainer -- a VBoxContainer lays out ALL its children in a
+	# vertical sequence, it has no concept of "float this one child behind
+	# the others as an overlay". That version rendered as an empty dark bar
+	# sitting above the text, not a background behind it.
+	var panel := PanelContainer.new()
+	panel.size_flags_horizontal = Control.SIZE_EXPAND_FILL
+	panel.custom_minimum_size = Vector2(0, 80)
+	var style := StyleBoxFlat.new()
+	style.bg_color = Color(0.15, 0.15, 0.15, 1.0)
+	style.content_margin_left = 12
+	style.content_margin_right = 12
+	style.content_margin_top = 8
+	style.content_margin_bottom = 8
+	panel.add_theme_stylebox_override("panel", style)
+
 	var card := VBoxContainer.new()
-	card.size_flags_horizontal = Control.SIZE_FILL
-	card.custom_minimum_size = Vector2(0, 80)
+	card.size_flags_horizontal = Control.SIZE_EXPAND_FILL
 	card.add_theme_constant_override("separation", 4)
+	panel.add_child(card)
 
 	var name_label := Label.new()
 	name_label.text = world_dict.get("name", "Unnamed World")
@@ -123,13 +139,4 @@ func _create_world_card(world_dict: Dictionary) -> Control:
 	card.add_child(desc_label)
 	card.add_child(meta_label)
 
-	# Subtle background for the card
-	var bg := ColorRect.new()
-	bg.color = Color(0.15, 0.15, 0.15, 1.0)
-	bg.size_flags_horizontal = Control.SIZE_FILL
-	bg.size_flags_vertical = Control.SIZE_FILL
-	bg.custom_minimum_size = Vector2(0, 80)
-	card.add_child(bg)
-	card.move_child(bg, 0)
-
-	return card
+	return panel