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/download.ts

@@ -33,7 +33,7 @@ import {
   FILEPATH_MARKER,
   DEST_MARKER
 } from './core/buildArgs'
-import { describeDownloadError } from './log'
+import { describeDownloadError, stderrDetail } from './log'
 import { addErrorLog } from './errorlog'
 import {
   STALL_TIMEOUT_MS,
@@ -117,7 +117,12 @@ function notify(wc: WebContents, title: string, body: string, incognito = false)
   if (flashWin && !flashWin.isDestroyed() && !flashWin.isFocused()) flashWin.flashFrame(true)
 }
 
-function logFailure(opts: StartDownloadOptions, title: string | undefined, error: string): void {
+function logFailure(
+  opts: StartDownloadOptions,
+  title: string | undefined,
+  error: string,
+  detail?: string
+): void {
   // L136: incognito downloads are never recorded — not even a failure entry, which
   // would persist the title + URL in the user-facing diagnostics log.
   if (opts.incognito) return
@@ -127,6 +132,7 @@ function logFailure(opts: StartDownloadOptions, title: string | undefined, error
     url: opts.url,
     kind: opts.kind,
     error,
+    detail,
     occurredAt: Date.now()
   })
 }
@@ -532,7 +538,7 @@ function wireChildProcess(params: {
     } else {
       const msg = describeDownloadError(stderrTail) || `yt-dlp exited with code ${code}`
       send(wc, { type: 'error', id: opts.id, error: msg })
-      logFailure(opts, getTitle(), msg)
+      logFailure(opts, getTitle(), msg, stderrDetail(stderrTail))
       notify(
         wc,
         opts.incognito ? 'Download failed' : (getTitle() ?? 'Download failed'),