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/locales/en.json

@@ -434,6 +434,7 @@
     "confirm.turn_off_device": "Turn off this device?",
     "common.loading": "Loading...",
     "common.delete": "Delete",
+    "common.remove": "Remove",
     "common.edit": "Edit",
     "common.clone": "Clone",
     "common.none": "None",
@@ -1995,6 +1996,8 @@
     "asset.error.name_required": "Name is required",
     "asset.error.no_file": "Please select a file to upload",
     "asset.error.delete_failed": "Failed to delete asset",
+    "asset.error.play_failed": "Failed to play sound",
+    "asset.error.download_failed": "Failed to download asset",
     "asset.play": "Play",
     "asset.download": "Download",
     "asset.prebuilt": "Prebuilt",