Fix L23/L78/L79: diagnostics hint, empty-state copy, SponsorBlock defaults

L23: Diagnostics error list now shows "Showing 20 of N errors." when there
     are more than 20 logged errors.
L78: DownloadsView empty-state copy updated from "Paste a URL above to get
     started" to "Paste a URL above, then click Download" -- matches the UI.
L79: SponsorBlock default categories expanded from ['sponsor'] to
     ['sponsor', 'selfpromo', 'interaction'] -- sponsor-only was silently
     covering almost nothing; the three new defaults are universally unwanted.
     Existing SB tests updated to pass explicit categories instead of using defaults.

typecheck + 242 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CODE-AUDIT.md

@@ -422,7 +422,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   version/update/export/import — two patterns in one file.
 - [ ] **L22 — DownloadOptionsForm shows both audio + video controls** regardless of the chosen
   kind in the per-download override panel (audio format shown for a video download, etc.).
-- [ ] **L23 — Diagnostics shows first 20 errors** with no "showing 20 of N" hint (the copied
+- [x] **L23 — Diagnostics shows first 20 errors** with no "showing 20 of N" hint (the copied
   report includes all).
 - [ ] **L24 — `window.open(htmlUrl, '_blank')`** in SettingsView is the only `window.open` in the
   app (relies on the window-open handler); elsewhere external opens go through IPC/`shell`.
@@ -522,9 +522,9 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   `titleFromUrl` placeholder before metadata resolves ('YouTube video (id)').
 - [x] **L77 — ffmpeg/ffprobe versions have no re-check affordance** (load once; "not found" sticks
   with no retry) unlike the yt-dlp "Check version" button.
-- [ ] **L78 — `DownloadsView` empty-state copy** ("Paste a URL above to get started") doesn't match
+- [x] **L78 — `DownloadsView` empty-state copy** ("Paste a URL above to get started") doesn't match
   the actual two-step Fetch→Download affordance.
-- [ ] **L79 — SponsorBlock default categories = `['sponsor']` only** — enabling SB silently covers
+- [x] **L79 — SponsorBlock default categories = `['sponsor']` only** — enabling SB silently covers
   just sponsors until the user expands categories (expectation gap).
 
 *Round 4 (2026-06-29) — doc/code drift, build artifacts, edge cases:*

src/renderer/src/components/DownloadsView.tsx

@@ -133,7 +133,7 @@ export function DownloadsView(): React.JSX.Element {
       {items.length === 0 ? (
         <div className={styles.empty}>
           <ArrowDownloadRegular fontSize={40} />
-          <Body1>Nothing queued yet. Paste a URL above to get started.</Body1>
+          <Body1>Nothing queued yet. Paste a URL above, then click Download.</Body1>
         </div>
       ) : (
         <VirtualList

src/renderer/src/components/SettingsView.tsx

@@ -1008,6 +1008,11 @@ export function SettingsView(): React.JSX.Element {
                 <Caption1 className={errText.errorPre}>{e.error}</Caption1>
               </div>
             ))}
+            {errorEntries.length > 20 && (
+              <Caption1 className={styles.hint}>
+                Showing 20 of {errorEntries.length} errors.
+              </Caption1>
+            )}
           </div>
         )}
       </Card>

src/shared/ipc.ts

@@ -217,7 +217,7 @@ export const DEFAULT_DOWNLOAD_OPTIONS: DownloadOptions = {
   autoSubtitles: false,
   sponsorBlock: false,
   sponsorBlockMode: 'remove',
-  sponsorBlockCategories: ['sponsor'],
+  sponsorBlockCategories: ['sponsor', 'selfpromo', 'interaction'],
   embedChapters: false,
   splitChapters: false,
   embedMetadata: true,

test/buildArgs.test.ts

@@ -220,13 +220,19 @@ describe('embedSubtitles', () => {
 
 describe('sponsorBlock', () => {
   it('remove → --sponsorblock-remove <cats> --force-keyframes-at-cuts', () => {
-    const argv = build(opts(), dlo({ sponsorBlock: true, sponsorBlockMode: 'remove' }))
+    const argv = build(
+      opts(),
+      dlo({ sponsorBlock: true, sponsorBlockMode: 'remove', sponsorBlockCategories: ['sponsor'] })
+    )
     expect(hasSeq(argv, '--sponsorblock-remove', 'sponsor', '--force-keyframes-at-cuts')).toBe(true)
     expect(argv).not.toContain('--sponsorblock-mark')
   })
 
   it('mark → --sponsorblock-mark <cats>, no keyframe forcing', () => {
-    const argv = build(opts(), dlo({ sponsorBlock: true, sponsorBlockMode: 'mark' }))
+    const argv = build(
+      opts(),
+      dlo({ sponsorBlock: true, sponsorBlockMode: 'mark', sponsorBlockCategories: ['sponsor'] })
+    )
     expect(hasSeq(argv, '--sponsorblock-mark', 'sponsor')).toBe(true)
     expect(argv).not.toContain('--sponsorblock-remove')
     expect(argv).not.toContain('--force-keyframes-at-cuts')