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>

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,