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>

scenes/world_list.tscn

@@ -11,14 +11,17 @@ grow_vertical = 2
 script = ExtResource("1_wl")
 
 [node name="VBoxContainer" type="VBoxContainer" parent="."]
-layout_mode = 3
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
 theme_override_constants/separation = 8
-size_flags_horizontal = 4
-size_flags_vertical = 4
 
 [node name="HeaderHBox" type="HBoxContainer" parent="VBoxContainer"]
 layout_mode = 3
-size_flags_horizontal = 4
+size_flags_horizontal = 3
 theme_override_constants/separation = 16
 
 [node name="BackButton" type="Button" parent="VBoxContainer/HeaderHBox"]
@@ -30,7 +33,7 @@ size_flags_horizontal = 0
 layout_mode = 3
 text = "Browse Worlds"
 horizontal_alignment = 1
-size_flags_horizontal = 4
+size_flags_horizontal = 3
 theme_override_font_sizes/font_size = 28
 
 [node name="RefreshButton" type="Button" parent="VBoxContainer/HeaderHBox"]
@@ -40,15 +43,14 @@ size_flags_horizontal = 0
 
 [node name="ScrollContainer" type="ScrollContainer" parent="VBoxContainer"]
 layout_mode = 3
-size_flags_horizontal = 4
-size_flags_vertical = 4
+size_flags_horizontal = 3
+size_flags_vertical = 3
 horizontal_mode = 2
 vertical_mode = 2
 
 [node name="WorldList" type="VBoxContainer" parent="VBoxContainer/ScrollContainer"]
 layout_mode = 3
-size_flags_horizontal = 4
-size_flags_vertical = 4
+size_flags_horizontal = 3
 theme_override_constants/separation = 8
 
 [node name="LoadingLabel" type="Label" parent="VBoxContainer"]