Float the selection bar over the grid instead of above it

Selecting the first photograph inserted a 40px row into the view's
vertical flow, so every cell in the grid moved down by it. The act of
selecting shifted the thing being selected out from under the finger —
and a second tap aimed at the neighbour landed on the row below it,
which is the worst possible response to a gesture whose whole job is to
say "this one".

The bar is a floating one now, at the foot of the view. Nothing above it
is re-laid out, so selecting changes what is drawn and never where.

The grid's viewport grows by the same 40px while the bar is there rather
than the Flickable shrinking, which is what keeps that change invisible
too: every cell stays exactly where it was and there is simply further to
scroll, so the last row can be brought clear of the bar instead of being
trapped under it.

It also swallows presses that land on it. A bar floating over the grid is
a bar a thumb can reach for and miss into.

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

ui/dr-ui/src/gesture_book.rs

@@ -85,20 +85,6 @@ pub const GESTURES: &[Gesture] = &[
         pointer: "Open the People chip on the filter bar, click each name, then switch the chip beside them to \"all of them\"",
         keys: "",
     },
-    Gesture {
-        title: "Drop the selection but keep selecting",
-        section: "Library grid",
-        touch: "Press Clear in the selection strip",
-        pointer: "Press Clear in the selection strip",
-        keys: "",
-    },
-    Gesture {
-        title: "Select everything the grid is showing",
-        section: "Library grid",
-        touch: "While selecting, press \"Select all\"",
-        pointer: "While selecting, press \"Select all\"",
-        keys: "",
-    },
     Gesture {
         title: "Resize the thumbnails",
         section: "Library grid",
@@ -127,4 +113,18 @@ pub const GESTURES: &[Gesture] = &[
         pointer: "Hover the cell, then click a star",
         keys: "0 to 5 on the selection",
     },
+    Gesture {
+        title: "Drop the selection but keep selecting",
+        section: "Library grid",
+        touch: "Press Clear in the selection strip",
+        pointer: "Press Clear in the selection strip",
+        keys: "",
+    },
+    Gesture {
+        title: "Select everything the grid is showing",
+        section: "Library grid",
+        touch: "While selecting, press \"Select all\"",
+        pointer: "While selecting, press \"Select all\"",
+        keys: "",
+    },
 ];