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.

src/viewport/TextureLibrary.ts

@@ -27,11 +27,8 @@ export class TextureLibrary {
    * the last one drawn wins; upload avoids that by reusing the existing asset for a known file.
    */
   get(ref: TextureRef): Texture | null {
-    const texture = this.loaded.get(ref.file);
-    if (!texture) {
-      this.startLoading(ref.file);
-      return null;
-    }
+    const texture = this.load(ref.file);
+    if (!texture) return null;
     const mode = ref.smooth ? 'linear' : 'nearest';
     if (texture.source.scaleMode !== mode) {
       texture.source.scaleMode = mode;
@@ -42,6 +39,20 @@ export class TextureLibrary {
     return texture;
   }
 
+  /**
+   * The picture if it is in memory, otherwise null while it loads – without touching the scale
+   * mode, which belongs to whoever wears the texture. A thumbnail must not decide the filter of
+   * a picture the scene is drawing with.
+   */
+  load(file: string): Texture | null {
+    const texture = this.loaded.get(file);
+    if (!texture) {
+      this.startLoading(file);
+      return null;
+    }
+    return texture;
+  }
+
   /** The picture if it is in memory, without starting a load or touching the scale mode. */
   peek(file: string): Texture | null {
     return this.loaded.get(file) ?? null;