fix(shell): dark DockButton theme variation for the system dock

DarkPanel is a PanelContainer-base variation (only a `panel`
stylebox); a Button looks up normal/hover/pressed, which don't exist
on it, so SystemDock's menu buttons rendered as default gray Godot
buttons instead of mock-faithful dark nav buttons.

Add a dedicated Button-base DockButton variation (Palette-only
colours, dock-dark normal/hover/pressed styleboxes), point
system_dock.gd's menu buttons at it, and regenerate the committed
game_theme.tres artifact from the builder.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scripts/theme/theme_keys.gd

@@ -9,6 +9,7 @@ const PRIMARY_CTA := &"PrimaryCTA"
 const TAB := &"Tab"
 const TAB_ACTIVE := &"TabActive"
 const CHIP := &"Chip"
+const DOCK_BUTTON := &"DockButton"
 const ITEM_TILE := &"ItemTile"
 const ITEM_TILE_EMPTY := &"ItemTileEmpty"
 const PARCHMENT_CARD := &"ParchmentCard"
@@ -28,6 +29,7 @@ const ALL := {
 	TAB: "Button",
 	TAB_ACTIVE: "Button",
 	CHIP: "Button",
+	DOCK_BUTTON: "Button",
 	ITEM_TILE: "PanelContainer",
 	ITEM_TILE_EMPTY: "PanelContainer",
 	PARCHMENT_CARD: "PanelContainer",