Offer the pictures the server already holds when adding a texture

"+ → Texture" opened the machine's file dialog, so a picture that was uploaded once
but lost its asset – an undone import, a deleted asset – was unreachable from the
editor while still sitting on the server. Nothing collects those files, by design.

It now opens a grid of every picture the server holds, with thumbnails: pick one and
"Use image" makes a texture asset on that file, "Upload…" is the way in for bytes the
server does not have yet and lands in the same import as a drop. A tile captioned with
an asset name is one this project already wears – picking it selects that asset instead
of making a second one, since two assets on one file would fight over `filter`.

- `GET ?r=textures` now reports `width`/`height`; a texture asset holds the pixel size,
  and reading it off the GPU texture would tie the asset's correctness to load timing.
- `TextureLibrary.load(file)` loads without touching `scaleMode` – a thumbnail must not
  restage the filter of a picture the scene is drawing with.
- The grid geometry lives in `texture/imageGrid.ts` as pure functions; inside the widget
  no test could reach it.
- The part after an upload is now one function shared by the drop import and the picker,
  so "one file = one asset" is stated in a single place.

12 new E2E checks; the strongest one undoes an import, leaving the file on the server
with no asset naming it, and picks exactly that file back out of the dialog.

api/lib/TextureStore.php

@@ -10,8 +10,9 @@ declare(strict_types=1);
  * of embedding megabytes of base64, and deleting a texture asset is undoable for free
  * because nothing here is removed when it stops being referenced.
  *
- * The flip side is that files outlive the projects that used them; `tools/gc-textures.mjs`
- * collects the unreferenced ones.
+ * The flip side is that files outlive the projects that used them: nothing collects the
+ * unreferenced ones. They stay pickable instead – `GET ?r=textures` is what the editor's
+ * image picker offers, so a picture one project dropped is still there for the next.
  */
 final class TextureStore
 {
@@ -91,8 +92,15 @@ final class TextureStore
         return ['file' => $file, 'width' => $width, 'height' => $height, 'bytes' => strlen($bytes)];
     }
 
-    /** Every stored file, newest first – the input of the garbage collector. */
-    /** @return list<array{file:string,bytes:int,updatedAt:string}> */
+    /**
+     * Every stored file, newest first – what the image picker offers.
+     *
+     * The pixel size is read from each header rather than left to the client: a texture asset
+     * records the size, so a picker that only knew the file name could not create one without
+     * decoding the picture first. An unreadable file reports 0 × 0 and the picker skips it.
+     *
+     * @return list<array{file:string,width:int,height:int,bytes:int,updatedAt:string}>
+     */
     public function list(): array
     {
         $items = [];
@@ -101,8 +109,11 @@ final class TextureStore
             if (!self::isValidName($file)) {
                 continue;
             }
+            $size = @getimagesize($path);
             $items[] = [
                 'file' => $file,
+                'width' => (int) ($size[0] ?? 0),
+                'height' => (int) ($size[1] ?? 0),
                 'bytes' => (int) filesize($path),
                 'updatedAt' => gmdate('c', (int) filemtime($path)),
             ];