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/login_screen.gd

@@ -65,6 +65,13 @@ var compact_layout := false
 var layout_updating := false
 
 
+func _notification(what: int) -> void:
+	if what == NOTIFICATION_APPLICATION_FOCUS_OUT:
+		var focus_owner := get_viewport().gui_get_focus_owner()
+		if focus_owner != null:
+			focus_owner.release_focus()
+
+
 func _ready() -> void:
 	auth_base_url = EndpointConfig.resolve(AUTH_BASE, "/api/Auth")
 	set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
@@ -851,7 +858,8 @@ func _on_steam_status(result: int, code: int, _headers: PackedStringArray, body:
 		"requires_account":
 			poll_timer.stop()
 			_close_steam_popup()
-			external_verified_email = str(data.get("email", ""))
+			var verified_email_value: Variant = data.get("email")
+			external_verified_email = "" if verified_email_value == null else str(verified_email_value)
 			steam_email_input.text = external_verified_email
 			steam_email_input.editable = external_verified_email.is_empty()
 			_set_mode("steam")