Fix invisible divider/caption colors on the study card's first paint

refresh_theme_controls() colored the divider and the ANSWER/KEY
TAKEAWAY captions, but it only ran on an actual theme toggle - never
at initial build. Confirmed live: the divider was invisible (default
white on a white card) and the captions rendered as plain body text
instead of the intended accent color until you toggled dark mode
once. Call refresh_theme_controls() once at the end of _build() so
the correct colors apply immediately, which also fixes the same
latent gap in empty_label's color.

scripts/ui/flashcards_study.gd

@@ -180,7 +180,8 @@ func _build() -> void:
 	undo_button.visible = false
 	undo_button.pressed.connect(_undo)
 	root.add_child(undo_button)
-	_style_ghost_button(undo_button, UIThemeService.is_dark_mode())
+
+	refresh_theme_controls()
 
 
 func _caption_label(text: String) -> Label: