feat(diagnostics): persist full stderr detail behind a "Show details" toggle

describeDownloadError condenses a postprocessing failure to one line for
the queue/notification, discarding the rest of yt-dlp's stderr. The
persisted error log now also keeps the fuller non-noise transcript
(stderrDetail) as an optional `detail` field, and the Diagnostics card
gets a collapsible "Show details" expander per row so a failure like
"Postprocessing: Conversion failed!" can be inspected -- and copied
into a bug report -- without re-running the download from a terminal.

src/main/core/validation.ts

@@ -74,7 +74,8 @@ export function isValidErrorLogEntry(o: unknown): o is ErrorLogEntry {
     (e.kind === 'video' || e.kind === 'audio') &&
     typeof e.error === 'string' &&
     typeof e.occurredAt === 'number' &&
-    isOptionalString(e.title)
+    isOptionalString(e.title) &&
+    isOptionalString(e.detail)
   )
 }