Fix manage handlers ignoring multipart form bodies

The /manage page posts with FormData, i.e. multipart/form-data, but the edit,
delete, tags, restore and empty-trash handlers parsed with r.ParseForm, which
only reads urlencoded bodies. So every field came back empty: delete returned
200 with deleted:0 and moved nothing, edits saved nothing — while curl -d
(urlencoded) worked, which is why tests and manual checks missed it.

Parse with ParseMultipartForm (tolerating ErrNotMultipart so urlencoded still
works), via a shared parseForm helper used by all manage handlers. handleUpload
already did this. Also stop the UI reporting deleted:0 / updated:0 as success,
and add a multipart regression test.

templates/manage.html

@@ -212,7 +212,9 @@
       const form = new FormData();
       form.append('slug', slug);
       try {
-        await post('/api/delete', form);
+        const res = await post('/api/delete', form);
+        const j = await res.json().catch(() => ({}));
+        if (!j.deleted) { flash('Not deleted — "' + slug + '" not found on the server', false); return; }
         el.remove();
         flash('Moved ' + slug + ' to trash', true);
         refreshBulk();
@@ -233,8 +235,10 @@
       const form = new FormData();
       form.append('slugs', slugs.join(','));
       try {
-        await post('/api/delete', form);
-        flash('Moved ' + slugs.length + ' to trash', true);
+        const res = await post('/api/delete', form);
+        const j = await res.json().catch(() => ({}));
+        if (!j.deleted) { flash('Nothing deleted — slugs not found on the server', false); return; }
+        flash('Moved ' + j.deleted + ' to trash', true);
         await load();
       } catch (err) { flash('Delete failed: ' + err.message, false); }
     });
@@ -247,9 +251,11 @@
       form.append('slugs', slugs.join(','));
       form.append(label, val);
       try {
-        await post('/api/tags', form);
+        const res = await post('/api/tags', form);
+        const j = await res.json().catch(() => ({}));
+        if (!j.updated) { flash('No posts updated', false); return; }
         document.getElementById(field).value = '';
-        flash('Updated tags on ' + slugs.length + ' post(s)', true);
+        flash('Updated tags on ' + j.updated + ' post(s)', true);
         await load();
       } catch (err) { flash('Tag update failed: ' + err.message, false); }
     }