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.

test/log.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { cleanError, describeDownloadError } from '../src/main/log'
+import { cleanError, describeDownloadError, stderrDetail } from '../src/main/log'
 
 describe('cleanError', () => {
   it('returns the last ERROR line, stripped of its prefix', () => {
@@ -76,3 +76,38 @@ describe('describeDownloadError', () => {
     expect(describeDownloadError('')).toBe('')
   })
 })
+
+describe('stderrDetail', () => {
+  it('keeps the full non-noise transcript for the Diagnostics "Show details" expander', () => {
+    const stderr = [
+      '[info] qkUTB65OfAc: Downloading 1 format(s): 137+251',
+      '[SponsorBlock] Found 1 segments in the SponsorBlock database',
+      '[ModifyChapters] Re-encoding "video.mp4" with appropriate keyframes',
+      'ERROR: Postprocessing: Conversion failed!'
+    ].join('\n')
+    const detail = stderrDetail(stderr)
+    expect(detail).toContain('[SponsorBlock] Found 1 segments')
+    expect(detail).toContain('[ModifyChapters] Re-encoding')
+    expect(detail).toContain('ERROR: Postprocessing: Conversion failed!')
+  })
+
+  it('strips [download] progress ticks and "Deleting original file" bookkeeping', () => {
+    const stderr = [
+      '[download]  42.0% of   23.21MiB at   15.11MiB/s ETA 00:00',
+      'Deleting original file video.f251.webm (pass -k to keep)',
+      '[ModifyChapters] Re-encoding "video.mp4" with appropriate keyframes',
+      'ERROR: Postprocessing: Conversion failed!'
+    ].join('\n')
+    const detail = stderrDetail(stderr)
+    expect(detail).not.toMatch(/\[download\]/)
+    expect(detail).not.toContain('Deleting original file')
+  })
+
+  it('returns undefined when there is nothing beyond a single headline', () => {
+    expect(stderrDetail('ERROR: Postprocessing: Conversion failed!')).toBeUndefined()
+  })
+
+  it('returns undefined for blank stderr', () => {
+    expect(stderrDetail('')).toBeUndefined()
+  })
+})