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/editor/debugHook.ts

@@ -4,6 +4,7 @@ import { findObjectHolder } from '../core/model/Project';
 import { Button, ui, Widget } from '../ui';
 import { textureLibrary } from '../viewport/TextureLibrary';
 import type { CentreTab, Editor } from './Editor';
+import { currentImagePicker } from './texture/ImagePicker';
 import type { EditorState } from './state';
 
 /**
@@ -168,6 +169,9 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
       center: () => centreOf(graphEditor.canvas),
       pickerWidth: () => graphEditor.pickerWidth,
     },
+    /** The open image picker: what it offers, what is picked, and where a tile sits. */
+    imagePicker: () => currentImagePicker()?.debugInfo() ?? null,
+    imagePickerTile: (file: string) => currentImagePicker()?.tileGlobal(file) ?? null,
     texture: {
       current: () => textureEditor.currentAssetId,
       pickerWidth: () => textureEditor.pickerWidth,