feat: animated GIF covers with config toggle and focus-based pausing

- Add settings menu with 'Animate GIF covers' toggle (default: off)
- When enabled: extract all GIF frames via ffmpeg, build AnimatedTexture
- When disabled: extract first frame only (static ImageTexture)
- AnimatedTexture starts paused — unpauses only when card has focus
- Focus-based pausing via gui_focus_changed signal (walks up to GameCard)
- Parse GIF89a frame delays for accurate animation speed
- Cache invalidation when setting changes (no restart needed)
- Cache AnimatedTextures in memory to avoid re-extraction
- Bump to v0.1.13

core/artprovider_settings.tscn

@@ -0,0 +1,20 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://plugins/itch-artprovider/core/artprovider_settings.gd" id="1"]
+[ext_resource type="PackedScene" uid="uid://d1qb7euwlu7bh" path="res://core/ui/components/toggle.tscn" id="2"]
+
+[node name="ArtProviderSettings" type="VBoxContainer"]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+theme_override_constants/separation = 10
+script = ExtResource("1")
+
+[node name="AnimatedGifsToggle" parent="." instance=ExtResource("2")]
+unique_name_in_owner = true
+layout_mode = 2
+text = "Animate GIF covers"
+description = "Extract all frames from animated GIF covers. Uses more memory but shows the full animation. Pauses when the card is not focused."
+button_pressed = false