Give the selection one bar instead of two ends of the window

"What can I do with these twelve photographs?" was answered in two
places. Six buttons in the header — Add to collection, Keywords,
Presets, Paste to N, Export N, Remove from collection — and four more
on the floating bar at the foot of the grid, beside the count that
says what they would act on.

The header half was the worse of the two. Those six appeared and
disappeared from the middle of the row as photographs were picked, so
Sync, Settings and everything beside them slid several hundred pixels
sideways at the exact moment a hand was already travelling toward one.
On a tablet the header scrolls sideways, so they were often not on
screen at all.

All six move to the bar. It now reads left to right as shaping the
selection — Clear, Select all, Select to… — then acting on it, with a
gap between the two thoughts. The header keeps only what belongs to
the library, and changes only when the library does.

Two consequences worth stating. The bar holds ten controls in the
worst case, so it scrolls sideways like every other row in this view,
for the reason set out on the header's Flickable: a layout given less
width than its children need overruns rather than shrinking, and the
buttons past the edge are simply gone. And the bar now stays up for a
running export whatever the selection has since become, because Cancel
export lived on a button that used to have its own `|| exporting`
escape hatch — the grid's viewport inset follows the same condition so
the last row of thumbnails is never trapped underneath.

`settings-summary` went with them: threaded from the window into the
grid and into HeaderActions, and never once drawn.

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

docs/gestures.md

@@ -54,7 +54,7 @@ The right match confidence is a property of your library, not of the model. "Wha
 
 Touch has no ctrl, so without a mode there is no way to select a second photograph — the first tap would open it. The hold is the fast way in and the button is the one that can be found.
 
-<sub>`ui/dr-ui/ui/library.slint:1274`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1172`</sub>
 
 ### Add or remove one photograph
 
@@ -63,7 +63,7 @@ Touch has no ctrl, so without a mode there is no way to select a second photogra
 
 While selecting, a tap never opens. That is the whole point of the mode: one meaning per gesture at a time. Press Done to get tap-to-open back.
 
-<sub>`ui/dr-ui/ui/library.slint:1283`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1181`</sub>
 
 ### Leave selecting
 
@@ -71,7 +71,7 @@ While selecting, a tap never opens. That is the whole point of the mode: one mea
 - **Pointer** — Press Done in the header
 - **Keyboard** — Escape
 
-<sub>`ui/dr-ui/ui/library.slint:1291`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1189`</sub>
 
 ### Select a range
 
@@ -80,7 +80,7 @@ While selecting, a tap never opens. That is the whole point of the mode: one mea
 
 This replaced a double tap, which had no visible state and could take forty photographs by accident. The run is resolved by the catalog rather than by what is on screen, so the grid can scroll between the two taps — the ranges that hurt on a tablet are longer than a screenful, which is exactly where a finger sweep runs out.
 
-<sub>`ui/dr-ui/ui/library.slint:1352`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1250`</sub>
 
 ### Find photographs with two people in them
 
@@ -89,7 +89,7 @@ This replaced a double tap, which had no visible state and could take forty phot
 
 "Any of them" is a union and "all of them" is an intersection. The tray is where both terms and the choice between them live, because a filter belongs on the filter bar.
 
-<sub>`ui/dr-ui/ui/library.slint:2093`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1936`</sub>
 
 ### Resize the thumbnails
 
@@ -98,7 +98,7 @@ This replaced a double tap, which had no visible state and could take forty phot
 
 There is no wheel on a tablet, so without the pinch the cell size could only be changed by a control a finger cannot reach.
 
-<sub>`ui/dr-ui/ui/library.slint:2719`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2562`</sub>
 
 ### File photographs in a collection
 
@@ -107,7 +107,7 @@ There is no wheel on a tablet, so without the pinch the cell size could only be
 
 The selection is what the drag carries, which is why selecting several is worth the mode: forty photographs file in one gesture.
 
-<sub>`ui/dr-ui/ui/library.slint:2878`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2725`</sub>
 
 ### Open a photograph
 
@@ -116,7 +116,7 @@ The selection is what the drag carries, which is why selecting several is worth
 
 A tap opens; a tap that *moved* does not. Travel is what separates a deliberate tap from a hand brushing past, and it is the only thing that does: the two are the same length. An earlier version required the finger to dwell 120 ms instead, and that rejected ordinary taps — a real tap is often quicker than a brush.
 
-<sub>`ui/dr-ui/ui/library.slint:3133`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2980`</sub>
 
 ### Rate a photograph without opening it
 
@@ -126,7 +126,7 @@ A tap opens; a tap that *moved* does not. Travel is what separates a deliberate
 
 A star has to take the press without it also reaching the cell, or every rating throws the user into develop.
 
-<sub>`ui/dr-ui/ui/library.slint:3245`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3092`</sub>
 
 ### Drop the selection but keep selecting
 
@@ -135,7 +135,7 @@ A star has to take the press without it also reaching the cell, or every rating
 
 Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the next selection can start straight away.
 
-<sub>`ui/dr-ui/ui/library.slint:3808`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3713`</sub>
 
 ### Select everything the grid is showing
 
@@ -144,4 +144,4 @@ Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the ne
 
 A scoped grid of two hundred frames is two hundred taps otherwise, and "all of them, except those three" is a far more common shape than the taps it took to say it.
 
-<sub>`ui/dr-ui/ui/library.slint:3825`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3730`</sub>