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/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,