Let a date range be stated, and draw the axis at the scale it deserves

"Limit to range" did nothing, and the reason was not visible from the button.
It took its span from the timeline's zoom, which is zero until someone zooms —
so `zoomed_span` returned the whole library and the filter narrowed to
everything. The chip lit up and the grid did not change.

The range has ends now, shown and typed as `YYYY-MM-DD`. Seeding them from the
timeline is kept, because zooming to a fortnight and pressing the chip is the
fast path; the fields say which fortnight it landed on and let it be corrected.
Ends given backwards are swapped rather than refused — there is exactly one
range between two days — and the closing day is included, since "to the 5th"
means the whole of the 5th and a range ending at its midnight contains none of
it. `parse_date` refuses anything that is not a date rather than guessing at an
order, because the alternative is a library silently filtered to a span nobody
asked for.

The axis then follows the range. It used to keep drawing the full extent while
a range was on, because it was the only way back out; the typed ends are the
way back out now, so it is free to show what was asked about.

And bucket size is chosen by how many bars it makes rather than by fixed
cut-offs. Each zoom step halves the span, so under thresholds the bar count
halved with it until a boundary was crossed: fifteen years went 15 bars, 8,
then 46, 23, 11, and finally 6. Zooming in made the picture coarser, which is
the opposite of what zooming is for. Aiming at forty bars keeps the count in
the same neighbourhood at every level, and the test asserts the property
directly — halving a span never coarsens the bucket.

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

ui/dr-ui/ui/app.slint

@@ -455,6 +455,11 @@ export component AppWindow inherits Window {
     /// Whether the grid is narrowed to the timeline's visible span.
     in-out property <bool> library-range-active: false;
     callback library-toggle-date-range();
+    /// TRACES: FR-CAT-6
+    in property <string> library-range-from;
+    in property <string> library-range-to;
+    in property <bool> library-range-invalid: false;
+    callback library-range-edited(string, string);
     callback library-retry-connection();
 
     // --- pinning a collection offline (FR-NC-6a) ---
@@ -1314,6 +1319,10 @@ in property <bool> panel-visible: true;
                     toggle-local-only() => { root.library-toggle-local-only(); }
                     range-active: root.library-range-active;
                     toggle-date-range() => { root.library-toggle-date-range(); }
+                    range-from: root.library-range-from;
+                    range-to: root.library-range-to;
+                    range-invalid: root.library-range-invalid;
+                    range-edited(a, b) => { root.library-range-edited(a, b); }
 
                     collections-visible: root.collections-visible;
                     toggle-collections() => { root.toggle-collections(); }