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/backup.ts

@@ -0,0 +1,57 @@
+import { dialog, type BrowserWindow } from 'electron'
+import { readFileSync, writeFileSync } from 'fs'
+import { getSettings, setSettings } from './settings'
+import { listTemplates, replaceTemplates } from './templates'
+import type {
+  BackupExportResult,
+  BackupImportResult,
+  Settings,
+  CommandTemplate
+} from '@shared/ipc'
+
+interface BackupFile {
+  version: 1
+  settings: Settings
+  templates: CommandTemplate[]
+}
+
+export async function exportBackup(win: BrowserWindow | undefined): Promise<BackupExportResult> {
+  const res = await dialog.showSaveDialog(win!, {
+    defaultPath: 'aerofetch-backup.json',
+    filters: [{ name: 'JSON', extensions: ['json'] }]
+  })
+  if (res.canceled || !res.filePath) return { ok: false }
+  const payload: BackupFile = { version: 1, settings: getSettings(), templates: listTemplates() }
+  try {
+    writeFileSync(res.filePath, JSON.stringify(payload, null, 2))
+    return { ok: true, path: res.filePath }
+  } catch (e) {
+    return { ok: false, error: (e as Error).message }
+  }
+}
+
+export async function importBackup(win: BrowserWindow | undefined): Promise<BackupImportResult> {
+  const res = await dialog.showOpenDialog(win!, {
+    properties: ['openFile'],
+    filters: [{ name: 'JSON', extensions: ['json'] }]
+  })
+  if (res.canceled || !res.filePaths[0]) return { ok: false }
+
+  let parsed: unknown
+  try {
+    parsed = JSON.parse(readFileSync(res.filePaths[0], 'utf8'))
+  } catch (e) {
+    return { ok: false, error: `Could not read backup file: ${(e as Error).message}` }
+  }
+  if (!parsed || typeof parsed !== 'object') {
+    return { ok: false, error: 'Not a valid AeroFetch backup file.' }
+  }
+
+  // setSettings/replaceTemplates validate and sanitize every field, so a
+  // hand-edited or partial backup file degrades gracefully rather than
+  // corrupting the store.
+  const file = parsed as Partial<BackupFile>
+  if (file.settings && typeof file.settings === 'object') setSettings(file.settings)
+  if (Array.isArray(file.templates)) replaceTemplates(file.templates)
+  return { ok: true }
+}