Complete Phase C (custom commands & yt-dlp updater) and Phase D (library, UX & notifications)

Phase C had been implemented in the working tree but never committed:
named custom-command templates (CRUD + per-download override), command
preview, and the in-app yt-dlp self-updater.

Phase D adds: history search/kind-filter/multi-select/bulk-delete/
re-download; native OS notifications on completion/failure; a private/
incognito download mode that skips history; settings+template backup/
restore via JSON; and a persistent error log surfaced as a Diagnostics
report in Settings.

See ROADMAP.md for the full per-item breakdown.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/settings.ts

@@ -29,7 +29,10 @@ const DEFAULTS: Settings = {
   cookieSource: 'none',
   cookiesBrowser: 'chrome',
   restrictFilenames: false,
-  downloadArchive: false
+  downloadArchive: false,
+  customCommandEnabled: false,
+  defaultTemplateId: null,
+  notifyOnComplete: true
 }
 
 /** Fixed path for the --download-archive file; not user-configurable. */
@@ -115,8 +118,13 @@ export function setSettings(partial: Partial<Settings>): Settings {
       case 'useAria2c':
       case 'restrictFilenames':
       case 'downloadArchive':
+      case 'customCommandEnabled':
+      case 'notifyOnComplete':
         if (typeof value === 'boolean') s.set(key, value)
         break
+      case 'defaultTemplateId':
+        if (value === null || typeof value === 'string') s.set('defaultTemplateId', value)
+        break
       case 'cookieSource':
         if (value === 'none' || value === 'browser' || value === 'login') s.set(key, value)
         break