feat: custom file drop zone for asset upload modal; fix review issues

Replace plain <input type="file"> with a styled drag-and-drop zone
featuring hover/drag states, file info display, and remove button.

Fix 10 review issues: add 401 handling to upload, remove non-null
assertions, add missing i18n keys (common.remove, error.play_failed,
error.download_failed), normalize close button glyphs to &#x2715;,
i18n the dropzone aria-label, replace silent error catches with toast
notifications, use DataTransfer for cross-browser file assignment.

server/src/wled_controller/static/js/features/assets.ts

@@ -102,7 +102,10 @@ function initUploadDropzone(): void {
         dropzone.classList.remove('dragover');
         const files = e.dataTransfer?.files;
         if (files && files.length > 0) {
-            fileInput.files = files;
+            // Use DataTransfer to safely assign files cross-browser
+            const dt = new DataTransfer();
+            dt.items.add(files[0]);
+            fileInput.files = dt.files;
             showFile(files[0]);
         }
     });
@@ -183,9 +186,11 @@ async function _playAssetSound(assetId: string) {
         const blobUrl = URL.createObjectURL(blob);
         const audio = new Audio(blobUrl);
         audio.addEventListener('ended', () => URL.revokeObjectURL(blobUrl));
-        audio.play().catch(() => {});
+        audio.play().catch(() => showToast(t('asset.error.play_failed'), 'error'));
         _currentAudio = audio;
-    } catch { /* ignore playback errors */ }
+    } catch {
+        showToast(t('asset.error.play_failed'), 'error');
+    }
 }
 
 // ── Modal ──
@@ -281,13 +286,14 @@ export async function uploadAsset(): Promise<void> {
     if (params.toString()) url += `?${params.toString()}`;
 
     try {
+        // Use raw fetch for multipart — fetchWithAuth forces Content-Type: application/json
+        // which breaks the browser's automatic multipart boundary generation
         const headers = getHeaders();
-        delete headers['Content-Type']; // Let browser set multipart boundary
-        const res = await fetch(url, {
-            method: 'POST',
-            headers,
-            body: formData,
-        });
+        delete headers['Content-Type'];
+        const res = await fetch(url, { method: 'POST', headers, body: formData });
+        if (res.status === 401) {
+            throw new Error('Session expired');
+        }
         if (!res.ok) {
             const err = await res.json();
             throw new Error(err.detail || 'Upload failed');
@@ -365,8 +371,8 @@ export async function saveAssetMetadata(): Promise<void> {
             method: 'PUT',
             body: JSON.stringify({ name, description: description || null, tags }),
         });
-        if (!res!.ok) {
-            const err = await res!.json();
+        if (!res.ok) {
+            const err = await res.json();
             throw new Error(err.detail);
         }
 
@@ -407,11 +413,11 @@ export async function deleteAsset(assetId: string): Promise<void> {
 export async function restorePrebuiltAssets(): Promise<void> {
     try {
         const res = await fetchWithAuth('/assets/restore-prebuilt', { method: 'POST' });
-        if (!res!.ok) {
-            const err = await res!.json();
+        if (!res.ok) {
+            const err = await res.json();
             throw new Error(err.detail);
         }
-        const data = await res!.json();
+        const data = await res.json();
         if (data.restored_count > 0) {
             showToast(t('asset.prebuilt_restored').replace('{count}', String(data.restored_count)), 'success');
         } else {
@@ -439,7 +445,9 @@ async function _downloadAsset(assetId: string) {
         a.download = filename;
         a.click();
         URL.revokeObjectURL(blobUrl);
-    } catch { /* ignore */ }
+    } catch {
+        showToast(t('asset.error.download_failed'), 'error');
+    }
 }
 
 // ── Event delegation ──