feat(theme): parchment Button variation for clickable parchment rows

ThemeKeys.PARCHMENT_CARD is a PanelContainer-base variation (only a
`panel` stylebox) — a Button looks up normal/hover/pressed/disabled,
which don't exist there, so a Button styled with it renders unskinned.
Add PARCHMENT_BUTTON, a Button-base variation with those four
styleboxes plus an empty focus box, built from Palette colours only.
Same fix pattern as DOCK_BUTTON for the system dock. Regenerated
game_theme.tres from the builder.

client/scripts/theme/theme_keys.gd

@@ -10,6 +10,7 @@ const TAB := &"Tab"
 const TAB_ACTIVE := &"TabActive"
 const CHIP := &"Chip"
 const DOCK_BUTTON := &"DockButton"
+const PARCHMENT_BUTTON := &"ParchmentButton"
 const ITEM_TILE := &"ItemTile"
 const ITEM_TILE_EMPTY := &"ItemTileEmpty"
 const PARCHMENT_CARD := &"ParchmentCard"
@@ -30,6 +31,7 @@ const ALL := {
 	TAB_ACTIVE: "Button",
 	CHIP: "Button",
 	DOCK_BUTTON: "Button",
+	PARCHMENT_BUTTON: "Button",
 	ITEM_TILE: "PanelContainer",
 	ITEM_TILE_EMPTY: "PanelContainer",
 	PARCHMENT_CARD: "PanelContainer",