Offer the categories in the masks panel

Everything before this was reachable only from an example that writes PPMs.
This is the part a photographer can touch: a list under the subjects, click
one, get a mask layer for every pixel of that category.

## Under the subjects, and the order is the argument

Clicking the photograph is how a local adjustment usually starts, so the
things the model *found* come first. A category is the move you reach for
deliberately — grade the sky, not this one bird — and putting it second says
so without a word of explanation.

Coverage is shown for the same reason a subject's score is: it tells the
photographer whether a category is worth a click before they spend one
finding out.

## Not a new tab, which is what was asked for

A develop tab is derived from `Attribute`, not declared — the tabs exist
because operations claim an attribute, and no amount of Slint adds one. A
seventh attribute would have meant duplicating every adjustment once per
category, and the combinatorics get silly by the third.

Reached through masks instead, a category composes with every adjustment
that already exists, and inherits feather and falloff rather than needing
its own. What was described as "per-category sliders with feather and decay"
is exactly what this is; only the door is different.

## Verified as far as it can be here

Compiles, populates, round-trips, 560 dr-ui tests green. **Not clicked** —
synthetic input is blocked on this setup, so how it looks and feels is
unverified and wants a human at the window.

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

ui/dr-ui/ui/app.slint

@@ -1,6 +1,6 @@
 import { Theme } from "theme.slint";
 import { AdjustPanel, GeometryPanel, GroupStrip, ParamRow, TransferPanel, ViewMode } from "adjust.slint";
-import { GradientHandle, GradientHandles, HandleRole, MaskPanel, MaskRow, SubjectRow } from "masks.slint";
+import { CategoryRow, GradientHandle, GradientHandles, HandleRole, MaskPanel, MaskRow, SubjectRow } from "masks.slint";
 import { SpotHandle, SpotHandles, SpotPanel, SpotRole } from "spots.slint";
 import { CropOverlay } from "crop.slint";
 import { HistoryPanel, HistoryRow } from "history.slint";
@@ -962,6 +962,7 @@ export component AppWindow inherits Window {
 
     in property <[MaskRow]> mask-rows;
     in property <[SubjectRow]> subject-rows;
+    in property <[CategoryRow]> category-rows;
     in property <bool> segmented: false;
     in property <bool> segmenting: false;
     in property <bool> refining: false;
@@ -1045,6 +1046,7 @@ export component AppWindow inherits Window {
     callback mask-morph-radius-changed(string, float);
     callback add-gradient-mask(bool);
     callback add-subject-mask(int);
+    callback add-category-mask(string);
 
     /// Whether the develop column — image info, geometry, adjust — is shown.
     ///
@@ -2472,6 +2474,7 @@ in property <bool> panel-visible: true;
                                 enabled: root.adjust-enabled;
                                 masks: root.mask-rows;
                                 subjects: root.subject-rows;
+                                categories: root.category-rows;
                                 segmented: root.segmented;
                                 segmenting: root.segmenting;
                                 refining: root.refining;
@@ -2505,6 +2508,7 @@ in property <bool> panel-visible: true;
                                 }
                                 add-gradient(radial) => { root.add-gradient-mask(radial); }
                                 add-subject(i) => { root.add-subject-mask(i); }
+                                add-category(n) => { root.add-category-mask(n); }
                             }
 
                             if root.local-mode: Rectangle {