Export at an exact resolution, and offer the panels worth naming

Export sizing could bound an image but not fix it. Long edge, short edge
and percentage all preserve the aspect ratio by letting one dimension
fall where it may, which is right for most work and useless against a
display that accepts one resolution and rejects everything else — a
television's art mode, a digital frame, a wallpaper slot.

FR-EXP-3 has always listed both halves of the answer, and this adds them.
**Fit box** scales to fit inside a width and height, so nothing is thrown
away and the result is smaller than the box on one axis unless the crop
already matches it. **Fill box** scales to cover the box and cuts the
overhang off the middle, so the file is exactly the pixels asked for.

Fill is the only mode in the file that discards image data, so two things
about it are worth stating. The overhang comes off symmetrically: the
crop tool is where a photographer decides which part of a frame survives,
and this stage having an opinion of its own would fight it. And locking
the crop to the same ratio leaves nothing here to cut, which is the
workflow the two features are meant to be used in.

With upscaling off and a source too small to cover, a fill box keeps its
*shape* rather than falling back to the source's: exporting a 3:2 file
where 16:9 was asked for is silently wrong in exactly the way the mode
exists to prevent, so the box shrinks instead. The existing rule — clamp,
never fail — is otherwise unchanged.

Four panel sizes are offered as buttons beside the fields. Getting 3840 x
2160 by typing four digits twice is a step at which the mistake is
discovered after the upload rather than before it. They fill in the
numbers and nothing else, in particular not the fit/fill choice: both are
legitimate against a screen, and guessing would discard the edges of a
photograph for a user who wanted them. The list is panels rather than
platforms, because a screen has one exact pixel count for ever where
"what a photo site wants" would rot in the file.

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

ui/dr-ui/ui/settings.slint

@@ -165,6 +165,16 @@ export component SettingsPage inherits Rectangle {
     in-out property <int> sizing-value: 0;
     in property <bool> sizing-has-value: false;
     in property <string> sizing-unit: "px";
+    /// "Size value" for the modes that take one number, "Width" for the box
+    /// modes, where a second field sits below it.
+    in property <string> sizing-value-label: "Size value";
+    in-out property <int> sizing-height: 0;
+    in property <bool> sizing-has-height: false;
+    /// The panels offered as buttons, and which one the numbers match — `-1`
+    /// where they match none, so a preset never stays lit over fields that
+    /// have since been typed over.
+    in property <[string]> screen-labels;
+    in property <int> screen-selected: -1;
     in property <bool> allow-upscaling: false;
     in property <[string]> sharpening-labels;
     in property <int> sharpening-selected: 0;
@@ -205,6 +215,8 @@ export component SettingsPage inherits Rectangle {
     callback colour-picked(int);
     callback sizing-picked(int);
     callback sizing-value-changed(string);
+    callback sizing-height-changed(string);
+    callback screen-picked(int);
     callback upscaling-toggled(bool);
     callback sharpening-picked(int);
     callback template-changed(string);
@@ -653,13 +665,42 @@ export component SettingsPage inherits Rectangle {
                         // "Original" has none, and a field showing 0 beside it
                         // would invite the reading "zero pixels".
                         if root.sizing-has-value: TextRow {
-                            label: "Size value";
+                            label: root.sizing-value-label;
                             text: root.sizing-value;
                             unit: root.sizing-unit;
                             field-width: 90px;
                             accepted(t) => { root.sizing-value-changed(t); }
                         }
 
+                        // TRACES: FR-EXP-3
+                        // The second axis, for the box modes only.
+                        if root.sizing-has-height: TextRow {
+                            label: "Height";
+                            text: root.sizing-height;
+                            unit: "px";
+                            field-width: 90px;
+                            accepted(t) => { root.sizing-height-changed(t); }
+                        }
+
+                        // TRACES: FR-EXP-3
+                        // Panel sizes as buttons, because the numbers are the
+                        // whole difficulty: a television's art mode accepts one
+                        // resolution and rejects everything else, and getting
+                        // it by typing four digits twice is a step at which a
+                        // photographer discovers they were wrong only after the
+                        // upload.
+                        //
+                        // Alongside the fields rather than instead of them —
+                        // the presets are a shortcut to a pair of numbers, not
+                        // a replacement for being able to say any pair.
+                        if root.sizing-has-height: Segmented {
+                            label: "Screen";
+                            hint: "fills in both numbers";
+                            options: root.screen-labels;
+                            selected: root.screen-selected;
+                            picked(i) => { root.screen-picked(i); }
+                        }
+
                         Check {
                             label: "Allow upscaling";
                             hint: "Off, a request larger than the source exports "