Change soundtrack covers from the app and show the cover with the answer

Cover menu in the soundtrack popup ("Change cover"):
- Choose picture uploads a JPEG, PNG or WebP picture (up to 10 MB).
- Pick by ID uses a game's cover on SteamGridDB or an anime's on AniList.
- Remove cover deletes it, so the server searches for a new one.
- The server's reason is shown when it refuses a change.
After a change the new cover time is fetched, and the popup, the
statistics card and the quiz show the new cover.

Quiz: once the answer is shown, the soundtrack's cover shows to the left of
the titles. Cover and titles are centered together, left and right and up
and down, and each title keeps to two lines. Cover times come from the
soundtrack list, and songs keep their soundtrack's UUID, also in saves.

Requests can now send any body with its content type, and every 2xx answer
counts as success. Cover frames are 2:3 like the server's covers. Small
copies of real covers are used by the tests and screenshots.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

popups/SoundtrackStatsPopup.tscn

@@ -36,8 +36,8 @@ clip_children = 2
 layout_mode = 0
 offset_left = 20.0
 offset_top = 64.0
-offset_right = 164.0
-offset_bottom = 264.0
+offset_right = 160.0
+offset_bottom = 274.0
 mouse_filter = 2
 theme_type_variation = &"SoundtrackCover"
 
@@ -90,13 +90,22 @@ autowrap_mode = 3
 text_overrun_behavior = 3
 max_lines_visible = 3
 
-[node name="EditNamesButton" type="Button" parent="StatsPanel/Info"]
+[node name="InfoButtons" type="HBoxContainer" parent="StatsPanel/Info"]
+layout_mode = 2
+theme_override_constants/separation = 12
+
+[node name="EditNamesButton" type="Button" parent="StatsPanel/Info/InfoButtons"]
 unique_name_in_owner = true
 layout_mode = 2
-size_flags_horizontal = 0
 focus_mode = 0
 text = "Edit names"
 
+[node name="ChangeCoverButton" type="Button" parent="StatsPanel/Info/InfoButtons"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Change cover"
+
 [node name="ScrollContainer" type="ScrollContainer" parent="StatsPanel" unique_id=1213000005]
 unique_name_in_owner = true
 layout_mode = 0
@@ -180,3 +189,111 @@ unique_name_in_owner = true
 layout_mode = 2
 focus_mode = 0
 text = "Save"
+
+[node name="CoverEditor" type="VBoxContainer" parent="StatsPanel"]
+unique_name_in_owner = true
+visible = false
+layout_mode = 0
+offset_left = 20.0
+offset_top = 280.0
+offset_right = 680.0
+offset_bottom = 784.0
+theme_override_constants/separation = 10
+
+[node name="CoverEditorTitle" type="Label" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+theme_type_variation = &"Title"
+text = "Cover"
+
+[node name="CoverEditorHelp" type="Label" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+autowrap_mode = 3
+text = "The server finds covers by itself. When it found the wrong one, or none, choose a picture of your own or the cover of a game on SteamGridDB or an anime on AniList. The server keeps a cover set here until it is removed."
+
+[node name="PictureRow" type="HBoxContainer" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+theme_override_constants/separation = 12
+
+[node name="ChoosePictureButton" type="Button" parent="StatsPanel/CoverEditor/PictureRow"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Choose picture…"
+
+[node name="RemoveCoverButton" type="Button" parent="StatsPanel/CoverEditor/PictureRow"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Remove cover"
+
+[node name="SourceTitle" type="Label" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+theme_type_variation = &"HighlightMedium"
+text = "Pick by ID"
+
+[node name="SourceHelp" type="Label" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+theme_type_variation = &"TextMedium"
+autowrap_mode = 3
+text = "The ID is the number in the game's or anime's web address."
+
+[node name="SourceRow" type="HBoxContainer" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+theme_override_constants/separation = 8
+
+[node name="CoverSourceButton" type="OptionButton" parent="StatsPanel/CoverEditor/SourceRow"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+selected = 0
+item_count = 2
+popup/item_0/text = "SteamGridDB"
+popup/item_0/id = 0
+popup/item_1/text = "AniList"
+popup/item_1/id = 1
+
+[node name="CoverIdField" type="LineEdit" parent="StatsPanel/CoverEditor/SourceRow"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+placeholder_text = "ID"
+max_length = 50
+
+[node name="UseSourceButton" type="Button" parent="StatsPanel/CoverEditor/SourceRow"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Use"
+
+[node name="CoverEditorSpace" type="Control" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+size_flags_vertical = 3
+mouse_filter = 2
+
+[node name="CoverStatus" type="Label" parent="StatsPanel/CoverEditor"]
+unique_name_in_owner = true
+visible = false
+layout_mode = 2
+theme_type_variation = &"HighlightMedium"
+autowrap_mode = 3
+
+[node name="CoverEditorButtons" type="HBoxContainer" parent="StatsPanel/CoverEditor"]
+layout_mode = 2
+alignment = 2
+
+[node name="CloseCoverButton" type="Button" parent="StatsPanel/CoverEditor/CoverEditorButtons"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Done"
+
+[node name="PictureDialog" type="FileDialog" parent="."]
+unique_name_in_owner = true
+title = "Choose a cover picture"
+initial_position = 2
+size = Vector2i(800, 500)
+ok_button_text = "Open"
+file_mode = 0
+access = 2
+filters = PackedStringArray("*.jpg, *.jpeg, *.png, *.webp ; Pictures")
+use_native_dialog = true