draft the crop instead of saving every pointer move

Panning went berserk. Wired straight to MediaField, `onCropChange` fired on every
move, each move saved, each save reloaded the row, and the reload handed the crop
back as the last SAVED value — yanking the picture out from under the drag.

BlockMedia exists for exactly this and I went around it. It keeps the crop in a
local draft, saves once when the drag stops, and resets on the url and the shape
rather than on the crop — its own note says why, and the failure it describes is
the one I reproduced.

Also: an empty picture slot looked the same whether the carousel was still being
read or genuinely had nothing in it. `undefined` now means loading and `null`
means empty, and the line underneath says which.

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

web/sections/SubjectFields.tsx

@@ -4,7 +4,7 @@ import { Input } from '@/components/ui/input';
 import { Textarea } from '@/components/ui/textarea';
 import { RichText } from '../RichText';
 import { useAutosave } from '../useAutosave';
-import { MediaField } from '../MediaField';
+import { BlockMedia } from './BlockMedia';
 import { SHAPES } from './shapes';
 import type { SubjectRecord, SubjectSection } from './subjects';
 
@@ -106,18 +106,29 @@ export const SubjectFields = <T extends SubjectRecord>({
       */}
       <div className="space-y-1.5">
         <label className="text-xs text-muted-foreground">Main image</label>
-        <MediaField
+        {/*
-          value={cover}
+          `BlockMedia`, not `MediaField` directly.
-          onChange={(path) => save({ cover_url: path, ...(path ? {} : { cover_crop: null }) })}
+
+          It holds the crop in a local draft and saves once, when the drag stops. Wired straight to
+          `MediaField` this saved on every pointer move — and each save reloaded the row, which handed
+          the crop back as the last SAVED value and yanked the picture out from under the drag. That is
+          the exact failure the note at the top of BlockMedia describes, and the reason it exists.
+        */}
+        <BlockMedia
+          url={cover}
           crop={record.cover_crop}
-          onCropChange={(crop) => save({ cover_crop: crop })}
           aspect={aspect}
-          label="Choose the main image"
+          onUrl={(path) => save({ cover_url: path, ...(path ? {} : { cover_crop: null }) })}
+          onCrop={(crop) => save({ cover_crop: crop })}
         />
         <p className="text-[11px] text-muted-foreground">
           {record.cover_url
             ? 'Chosen for this one. Remove it to go back to the first picture in its carousel.'
-            : 'From the first picture in its carousel. Choose one to override that.'}
+            : firstPhoto === undefined
+              ? 'Reading its carousel…'
+              : firstPhoto
+                ? 'From the first picture in its carousel. Choose one to override that.'
+                : 'Its carousel is empty. Choose a picture, or add some in Carousels.'}
           {record.cover_shape === null && ' No shape is set on the page, so it draws at its own.'}
         </p>
       </div>

web/sections/SubjectPanel.tsx

@@ -122,7 +122,9 @@ export const SubjectPanel = <T extends SubjectRecord>({ section }: { section: Su
           record={selected}
           // What the cover falls back to when the row has not chosen one — the same first item the
           // grid below is already showing, so the two cannot disagree about which picture that is.
-          firstPhoto={photos?.[0]?.path ?? null}
+          // `undefined` while the carousel is still being read, `null` when it genuinely has none.
+          // The two look identical in an empty picture slot, and only one of them is a problem.
+          firstPhoto={photos === null ? undefined : (photos[0]?.path ?? null)}
           onSaved={() => void load()}
         />
       ) : null,