Keep the grid's place when another screen covers it

Opening Settings, Import or People and coming back landed at the top of
the library however deep in it you had been.

The grid is gated on an `if` in the markup, so every route away from it
destroys the subtree and rebuilds it. A Flickable being destroyed passes
its viewport through zero on the way out, and that reaches
`on_library_scrolled` looking exactly like the user having flung the grid
to the top. The handler already guarded against it -- but on
`show-library`, which means "the library rather than develop" and stays
true while any of those four screens replaces the window. So the guard
covered the develop route and none of the other three: `resume_at` was
overwritten with 0 on the way out, and the position was gone before
anything could restore it.

The condition the `if` is actually spelled with is now computed once, in
`app.slint`, and Rust reads that. The two cannot drift apart again
because there is only one of them.

That fixes the overwrite. The second half is that nothing replayed the
position on the way back in: `on_back_to_library` does it by hand, and
Settings, Import, People and the launch screen do not go through it.
Rather than teaching three more modules to call it, `scroll-to` is now
kept current on every scroll. It is read by `seek()`, which runs on a
token change and on `init`, so writing it without bumping the token
cannot move the grid on screen -- and is exactly what the next grid reads
when it is built.

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:1217`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1292`</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:1226`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1301`</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:1234`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1309`</sub>
 
 ### Pick a photograph up to drag it
 
@@ -80,7 +80,7 @@ While selecting, a tap never opens. That is the whole point of the mode: one mea
 
 A finger on a photograph might be starting a scroll, and for the first half-second the grid assumes it is. Holding says otherwise, and the ring is the grid saying it heard — from there the drag cannot be lost to a scroll. A mouse never waits: the cursor is precise enough that a sideways drag is unambiguous from the first pixel.
 
-<sub>`ui/dr-ui/ui/library.slint:1264`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1339`</sub>
 
 ### Select a range
 
@@ -89,7 +89,7 @@ A finger on a photograph might be starting a scroll, and for the first half-seco
 
 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:1329`</sub>
+<sub>`ui/dr-ui/ui/library.slint:1404`</sub>
 
 ### Find photographs with two people in them
 
@@ -98,7 +98,7 @@ 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:2011`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2086`</sub>
 
 ### Resize the thumbnails
 
@@ -107,7 +107,7 @@ 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:2664`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2739`</sub>
 
 ### File photographs in a collection
 
@@ -116,7 +116,7 @@ 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:2827`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2936`</sub>
 
 ### Open a photograph
 
@@ -125,7 +125,7 @@ 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:3095`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3204`</sub>
 
 ### Rate a photograph without opening it
 
@@ -135,7 +135,7 @@ 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:3215`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3324`</sub>
 
 ### Drop the selection but keep selecting
 
@@ -144,7 +144,7 @@ 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:3879`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4032`</sub>
 
 ### Select everything the grid is showing
 
@@ -153,4 +153,4 @@ 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:3896`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4049`</sub>