fix: remove group_name from platform_assets insert — column doesn't exist

The platform_assets table has no group_name column (unlike platform_theme).
This was causing all emoji uploads from admin to silently fail with HTTP 400.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin/branding.php

@@ -96,19 +96,11 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['asset'])) {
             $theme['assets'][$slot] = $publicUrl;
 
             // Save asset URL to Supabase platform_assets table (upsert)
-            $assetData = [
-                'id' => $slot,
-                'category' => 'custom',
-                'group_name' => 'player_app',
-                'label' => $slot,
-                'asset_url' => $publicUrl,
-                'dimensions' => $expectedW . 'x' . $expectedH
-            ];
             $existing = $sdb->get('platform_assets', ['id' => 'eq.' . $slot, 'select' => 'id', 'limit' => 1]);
             if (!empty($existing) && !isset($existing['error'])) {
                 $result = $sdb->update('platform_assets', ['asset_url' => $publicUrl, 'dimensions' => $expectedW . 'x' . $expectedH], ['id' => 'eq.' . $slot]);
             } else {
-                $result = $sdb->insert('platform_assets', $assetData);
+                $result = $sdb->insert('platform_assets', ['id' => $slot, 'category' => 'custom', 'label' => $slot, 'asset_url' => $publicUrl, 'dimensions' => $expectedW . 'x' . $expectedH]);
             }
             if (isset($result['error'])) {
                 error_log('[branding] platform_assets save failed: ' . json_encode($result));