Game picker and save browser: menu window shell, background warm-up

The main menu warms every ready game's mods up into the assembly cache in the
background (last played first), so picking a game only loads cached assemblies.
Game picker and save browser are scenes on the shared MenuWindowShell (also used by
the settings): caps title, close on the right, footer hint; the picker footer shows
the warm-up state, cards get framed covers with a letter placeholder and Continue as
the main action; the save browser shows 3x2 large cards with the game selector in
the header. new_game waits for the warm-up; menu_flow takes --shots.

src/MenuWindowShell.cs

@@ -0,0 +1,66 @@
+using System;
+using Cthangover.Core.UI.Theming;
+using Cthangover.Mods.Interface.Theming;
+using Godot;
+
+namespace Cthangover.BaseMenu
+{
+    /// <summary>
+    /// The look of the menu windows (settings, save browser, game picker) — the
+    /// style guide's panel shell: a 0.9 backdrop, the window frame, a header
+    /// with the caps title on the left and the ✕ on the right, 1px rules around
+    /// the body and a footer with a faint hint. The node layout lives in each
+    /// window's scene under the same names (<c>Backdrop</c>, <c>Window/Column/…</c>),
+    /// this class only paints it, so the three windows cannot drift apart.
+    /// </summary>
+    public static class MenuWindowShell
+    {
+        /// <summary>Header row: <c>Title</c>, optional controls, <c>CloseButton</c>.</summary>
+        public const string Header = "Window/Column/Header/Row/";
+
+        /// <summary>Body container of the window.</summary>
+        public const string Body = "Window/Column/Body/";
+
+        /// <summary>Footer row: <c>Hint</c> and optional buttons.</summary>
+        public const string Footer = "Window/Column/Footer/Row/";
+
+        /// <summary>
+        /// Paints the shell of <paramref name="root"/> and wires its close button to
+        /// <paramref name="close"/>; the title, the close tooltip and the footer
+        /// hint keep their texts (callers translate them).
+        /// </summary>
+        public static void Style(Control root, Action close)
+        {
+            CthangoverTheme.Apply(root);
+            root.GetNode<ColorRect>("Backdrop").Color = UiTokens.Backdrop;
+            root.GetNode<PanelContainer>("Window").AddThemeStyleboxOverride("panel", UiKit.Frame(UiTokens.RadiusLg, UiTokens.Surface0, UiTokens.Line));
+            root.GetNode<ColorRect>("Window/Column/RuleHeader").Color = UiTokens.Line;
+            root.GetNode<ColorRect>("Window/Column/RuleFooter").Color = UiTokens.Line;
+
+            var title = root.GetNode<Label>(Header + "Title");
+            title.AddThemeFontSizeOverride("font_size", UiTokens.FontTitle);
+            title.AddThemeColorOverride("font_color", UiTokens.InkBright);
+
+            var closeButton = root.GetNode<Button>(Header + "CloseButton");
+            CthangoverTheme.ApplyIcon(closeButton);
+            closeButton.TooltipText = TranslationServer.Translate("settings/back");
+            closeButton.Pressed += close;
+
+            StyleHint(root.GetNode<Label>(Footer + "Hint"));
+        }
+
+        /// <summary>Faint helper text: footer hints and notes under a setting.</summary>
+        public static void StyleHint(Label label)
+        {
+            label.AddThemeFontSizeOverride("font_size", UiTokens.FontBodySmall);
+            label.AddThemeColorOverride("font_color", UiTokens.Faint);
+        }
+
+        /// <summary>Section caption: small muted caps, as section titles everywhere in the kit.</summary>
+        public static void StyleSection(Label label)
+        {
+            label.AddThemeFontSizeOverride("font_size", UiTokens.FontSmall);
+            label.AddThemeColorOverride("font_color", UiTokens.Muted);
+        }
+    }
+}