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/errorlog.ts

@@ -0,0 +1,41 @@
+import { app } from 'electron'
+import { join } from 'path'
+import { readFileSync, writeFileSync, existsSync } from 'fs'
+import type { ErrorLogEntry } from '@shared/ipc'
+
+// Plain JSON in userData, same shape as history.ts. Persisted so a failure
+// report survives the queue item being cleared (Seal's "debug report").
+const MAX_ENTRIES = 200
+
+function errorLogFile(): string {
+  return join(app.getPath('userData'), 'errorlog.json')
+}
+
+export function listErrorLog(): ErrorLogEntry[] {
+  try {
+    if (!existsSync(errorLogFile())) return []
+    const data = JSON.parse(readFileSync(errorLogFile(), 'utf8'))
+    return Array.isArray(data) ? (data as ErrorLogEntry[]) : []
+  } catch {
+    return []
+  }
+}
+
+function save(entries: ErrorLogEntry[]): void {
+  try {
+    writeFileSync(errorLogFile(), JSON.stringify(entries.slice(0, MAX_ENTRIES), null, 2))
+  } catch {
+    /* best-effort; a read-only data dir just means no persisted error log */
+  }
+}
+
+export function addErrorLog(entry: ErrorLogEntry): ErrorLogEntry[] {
+  const entries = [entry, ...listErrorLog()]
+  save(entries)
+  return entries
+}
+
+export function clearErrorLog(): ErrorLogEntry[] {
+  save([])
+  return []
+}