Say which axis each export dimension is, in the one place that renders

The box modes put two numeric fields one above the other, and on screen
they are two anonymous numbers: `TextRow` draws its label behind the
field rather than above it, so "Width" and "Height" never appear. That
fault is older than this feature — the storage panel's cache sizes have
the same missing labels, and the single "Size value" row always did — and
it belongs in its own change rather than being fixed under cover of this
one.

But one unlabelled number is survivable and two are not, so the axis goes
where the page does render it: the unit. It reads "3840 px wide" above
"2160 px high", which is the sentence the user is trying to write anyway.

The `label` bindings stay correct and stay where they are, so this
becomes redundant rather than wrong the day `TextRow` is fixed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/settings_ui.rs

@@ -196,9 +196,15 @@ pub fn render(window: &AppWindow, controller: &SettingsController) {
     // `Original` carries no number, so the field beside the chips has nothing
     // to edit and is hidden rather than shown holding a meaningless zero.
     window.set_settings_sizing_has_value(s.export.sizing.value().is_some());
+    // TRACES: FR-EXP-3
+    // The unit carries the axis for the box modes, so that two numeric fields
+    // one above the other are not two anonymous numbers. It reads "3840 px
+    // wide" and "2160 px high", which is the sentence the user is trying to
+    // write anyway.
     window.set_settings_sizing_unit(
         match s.export.sizing {
             SizingMode::Percentage(_) => "%",
+            SizingMode::FitBox(..) | SizingMode::FillBox(..) => "px wide",
             _ => "px",
         }
         .into(),