fix(call): Response return-type can't be wrapped in Result

Previous commit had the command typed as Result<tauri::ipc::Response,
String>. Turns out that forces Tauri to JSON-serialise the variant
wrapper around the Response body — the frontend gets a JSON object
instead of the raw ArrayBuffer, the runtime check for byteLength fails,
and every thumbnail comes back as null.

Changed the return type to `tauri::ipc::Response` directly. Bad source
ids and capture failures now funnel into an empty byte buffer; the JS
side still detects "no thumbnail" via `byteLength === 0` so the
contract stays the same.

Frontend also widens the invoke-result typing to ArrayBuffer |
Uint8Array | number[] so an older WebView2 that happens to deserialise
as an array still works, and normalises into a plain ArrayBuffer
before constructing the Blob to sidestep a TS SharedArrayBuffer
incompatibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src-tauri/src/screen_sources.rs

@@ -83,22 +83,23 @@ pub fn capture_screen_source_thumbnail(source_id: String) -> Result<Option<Strin
 // expose via `URL.createObjectURL` — skips the base64-decode step
 // entirely and keeps the main thread responsive during fan-in.
 //
-// A capture failure (source vanished, permission denied) returns an empty
-// byte buffer rather than an error so the JS side gets a uniform contract
-// (ArrayBuffer always). Caller checks `byteLength === 0` to detect the
-// no-thumbnail case.
+// The return type MUST be `Response` directly (not `Result<Response, E>`):
+// a Result wrapper forces Tauri to JSON-serialise the variant so the
+// frontend gets a JSON object instead of raw bytes. Failures — bad id
+// format, capture errors, source vanished — all funnel into an empty
+// byte buffer; the caller treats `byteLength === 0` as the "no thumbnail"
+// signal.
 #[tauri::command]
-pub fn capture_screen_source_thumbnail_bytes(
-    source_id: String,
-) -> Result<tauri::ipc::Response, String> {
+pub fn capture_screen_source_thumbnail_bytes(source_id: String) -> tauri::ipc::Response {
     let bytes = if let Some(raw) = source_id.strip_prefix("screen:").and_then(strip_zero_suffix) {
         capture_monitor_bytes_by_id(raw).unwrap_or_default()
     } else if let Some(raw) = source_id.strip_prefix("window:").and_then(strip_zero_suffix) {
         capture_window_bytes_by_id(raw).unwrap_or_default()
     } else {
-        return Err(format!("unknown source id format: {source_id}"));
+        eprintln!("capture_screen_source_thumbnail_bytes: unknown id format: {source_id}");
+        Vec::new()
     };
-    Ok(tauri::ipc::Response::new(bytes))
+    tauri::ipc::Response::new(bytes)
 }
 
 fn strip_zero_suffix(s: &str) -> Option<&str> {