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/shared/ipc.ts

@@ -845,6 +845,13 @@ export interface ErrorLogEntry {
   url: string
   kind: MediaKind
   error: string
+  /**
+   * Fuller non-noise stderr context behind the Diagnostics card's "Show
+   * details" toggle — undefined when `error` already says everything (most
+   * failures), present for the generic "Postprocessing: Conversion failed!"
+   * wrapper where the useful detail is the tagged step lines that preceded it.
+   */
+  detail?: string
   /** epoch milliseconds */
   occurredAt: number
 }