Add textures for sprites

Images are uploaded to a content-addressed blob endpoint and stored as
data/textures/<sha256>.<ext>; the project only keeps the file name. The
sprite points at a texture asset through an ordinary property of the new
kind `asset`, so instance overrides, "Apply to container", "Reset to
asset", undo and `Set property` work without any extra code. The new kind
also replaces the ad-hoc asset picker the graph nodes faked as an enum.

Import goes through + -> Texture, drag & drop onto the canvas or Ctrl+V.
Images wider than 2048px are downscaled client-side and re-encoded to
WebP if needed, because nginx caps uploads at 1MB; an image that already
fits is uploaded byte for byte so pixel art is never resampled.

The sprite colour default changes from red to white, since colour tints
the texture multiplicatively and a tint of white shows an image as it is.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/store/http.ts

@@ -0,0 +1,22 @@
+/**
+ * The bit of HTTP handling both stores share. It exists because a failed response does not
+ * always come from our API: nginx caps request bodies at 1 MB and answers a larger one with
+ * its own HTML page, so there is no `{error}` object to read the reason from.
+ */
+
+/** Reasons for the statuses that can arrive without an API body of their own. */
+const STATUS_MESSAGES: Record<number, string> = {
+  400: 'Bad request',
+  403: 'Forbidden',
+  404: 'Not found',
+  413: 'Request too large',
+  500: 'Server error',
+  502: 'Server unavailable',
+  503: 'Server unavailable',
+  504: 'Server timed out',
+};
+
+/** What to tell the user about a failed response whose body is not our JSON – the status is all there is. */
+export function statusMessage(status: number, statusText: string): string {
+  return STATUS_MESSAGES[status] ?? (statusText || `HTTP ${status}`);
+}