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/log.ts

@@ -53,3 +53,20 @@ export function describeDownloadError(stderr: string): string {
   }
   return context.length > 0 ? `${headline} — ${context.join(' · ')}` : headline
 }
+
+/**
+ * Full non-noise stderr, for the persisted error log's "Show details" expander
+ * (Diagnostics card) — everything describeDownloadError condenses to one line,
+ * kept in full (progress ticks and delete-bookkeeping still stripped) so a bug
+ * report carries the real yt-dlp/ffmpeg transcript instead of a paraphrase.
+ * Returns undefined when there's nothing beyond a single line — the headline
+ * the caller already shows — so a plain, self-explanatory error doesn't grow a
+ * details toggle with nothing new in it.
+ */
+export function stderrDetail(stderr: string): string | undefined {
+  const lines = stderr
+    .split('\n')
+    .map((l) => l.trim())
+    .filter((l) => l && !CONTEXT_NOISE.test(l))
+  return lines.length > 1 ? lines.join('\n') : undefined
+}