Fix login/calculator theme bugs, gate flashcard button, hide system label pre-answer

- Steam-linked login no longer writes literal "<null>" into the email field
  (and no longer locks it read-only) when the verified identity has no email
- Login screen releases focus on app focus-out so the username caret stops
  blinking while the window is inactive
- Calculator display text stays legible in light mode; the LCD-style panel
  background is always dark, so its text should not follow the app theme
- Question player: gate the "save as flashcard" toolbar button behind
  Brand.is_uat_environment(), matching the nav item, so it's hidden in prod
- Hide the discipline/system label while a question is unanswered so it
  can't hint at the answer; reveal it once the explanation is shown
- Top-align question content instead of vertically centering it, with a
  20px top margin, so it sits closer to the progress indicator

scripts/ui/calculator_panel.gd

@@ -101,8 +101,8 @@ func refresh_theme_controls() -> void:
 		return
 	var dark := UIThemeService.is_dark_mode()
 	add_theme_stylebox_override("panel", Brand.content_card_box(dark, true, Brand.ROYAL_BLUE))
-	display_label.add_theme_color_override("font_color", Brand.text_color(dark))
-	operation_label.add_theme_color_override("font_color", Brand.muted_color(dark))
+	display_label.add_theme_color_override("font_color", Color.WHITE)
+	operation_label.add_theme_color_override("font_color", Brand.muted_color(true))
 	for button in keypad_buttons:
 		var accent := Brand.ROYAL_BLUE if str(button.get_meta("calculator_token", "")) in ["+", "−", "×", "÷", "="] else Brand.SLATE
 		button.add_theme_stylebox_override("normal", Brand.badge_box(dark, accent))