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>

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')