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/templates.ts

@@ -0,0 +1,58 @@
+import { app } from 'electron'
+import { join } from 'path'
+import { readFileSync, writeFileSync, existsSync } from 'fs'
+import type { CommandTemplate } from '@shared/ipc'
+
+// Plain JSON in userData, same shape as history.ts.
+const MAX_TEMPLATES = 100
+
+function templatesFile(): string {
+  return join(app.getPath('userData'), 'templates.json')
+}
+
+export function listTemplates(): CommandTemplate[] {
+  try {
+    if (!existsSync(templatesFile())) return []
+    const data = JSON.parse(readFileSync(templatesFile(), 'utf8'))
+    return Array.isArray(data) ? (data as CommandTemplate[]) : []
+  } catch {
+    return []
+  }
+}
+
+function save(templates: CommandTemplate[]): void {
+  try {
+    writeFileSync(templatesFile(), JSON.stringify(templates.slice(0, MAX_TEMPLATES), null, 2))
+  } catch {
+    /* best-effort; a read-only data dir just means no persisted templates */
+  }
+}
+
+function sanitize(t: CommandTemplate): CommandTemplate {
+  return {
+    id: String(t.id),
+    name: (t.name ?? '').trim() || 'Untitled command',
+    args: (t.args ?? '').trim()
+  }
+}
+
+/** Add a new template, or update an existing one (matched by id). */
+export function saveTemplate(template: CommandTemplate): CommandTemplate[] {
+  const clean = sanitize(template)
+  const templates = [clean, ...listTemplates().filter((t) => t.id !== clean.id)]
+  save(templates)
+  return templates
+}
+
+export function removeTemplate(id: string): CommandTemplate[] {
+  const templates = listTemplates().filter((t) => t.id !== id)
+  save(templates)
+  return templates
+}
+
+/** Replace the entire template list (backup restore) rather than merge-by-id. */
+export function replaceTemplates(templates: CommandTemplate[]): CommandTemplate[] {
+  const clean = templates.map(sanitize)
+  save(clean)
+  return clean
+}