Let the develop column ask how wide it needs to be

The column was 280px, a number chosen for a tablet, with 380px bolted on
later for a desktop. Both were guesses at how much room the widest row
inside needs, and a guess is what cannot work here: the mode strip is one
chip per attribute the *operation set declares*, so the row is generated
and no constant in app.slint can track it.

When the guess came up short the failure was not a tidy clip. The
Flickable inside the column never had its `viewport-width` set, so the
viewport took its content's preferred width, and a viewport wider than
its Flickable is *centred* in it — the same rule the note on the seam's
`x: 0` already records a few lines below. So the column lost half of each
edge rather than one of them: "HISTOGRAM" read "ISTOGRAM", "Straighten"
read "aighten", Copy sat centred while Paste ran off the far side. It
looked like a rendering fault and it was an alignment one.

So the column asks instead of guessing. Every panel that can appear in it
— image, histogram, geometry, settings transfer, masks, repairs, adjust,
history — now publishes a `content-width`: how wide it has to be before
it starts clipping itself, read off its own layout rather than asserted.
Each declares that as its `min-width` too, and that is what makes the
aggregation automatic: `column` is a layout, so it already reports the
largest minimum among its children, and it does so for the panels that
come and go with the mode as well, which live inside `if`s and cannot be
named from outside. Grep `content-width` in ui/dr-ui/ui to see every
panel with a say in the answer. The mode strip is named explicitly only
because it is pinned outside that layout, so nothing else measures it.

There is no floor left. A floor is one more guess and the panels state
their own minimums now. The only thing still above the measurement is
`panel-max-width`, which is not a size but a policy — a column may not
take the window from the photograph it exists to serve — and it comes
from Rust beside `layout-class` because a width read from `root.width`
inside the layout that `root.width` depends on is a binding loop.

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

ui/dr-ui/ui/history.slint

@@ -115,8 +115,16 @@ export component HistoryPanel inherits Rectangle {
     // No explicit height either, for the same reason `AdjustPanel` and
     // `MaskPanel` declare none: the row count changes as the photographer
     // works, and a height pinned to a layout's preferred size is one more
+    /// TRACES: FR-UI-2
+    /// How wide this panel has to be before it starts clipping itself. The
+    /// develop column is the largest of these and nothing else; publishing it
+    /// as `min-width` too is what makes the enclosing layout aggregate them
+    /// without this panel having to be referenced by name.
+    out property <length> content-width: layout.preferred-width;
+    min-width: root.content-width;
+
     // thing that has to keep up with a repeater.
-    VerticalLayout {
+    layout := VerticalLayout {
         padding: Theme.gap;
         spacing: Theme.gap-sm;
         alignment: start;