security: harden command exec, IPC, deep-link, cookies, and persistence

Seven-tier security audit of the main process, each finding fixed with a
regression test. Typecheck (node + web) clean; unit tests 106 -> 140.

- Tier 1 (command exec/argv): allowlist the yt-dlp --update-to channel
  (blocks arbitrary-binary-install RCE); gate per-download extraArgs behind
  the customCommandEnabled consent flag in main (blocks --exec RCE); resolve
  taskkill/schtasks by absolute System32 path; validate per-download
  outputDir; normalize the URL in assertHttpUrl and use it at every spawn.
- Tier 2 (input validation): fix isSafeFilenameTemplate drive-relative
  ('C:foo') and Windows dotted-'..' traversal bypasses; percent-encode the
  untrusted id in entryUrl; catch reserved device names with extensions in
  sanitizeDirSegment.
- Tier 3 (fs/backup): drop malformed template rows in importBackup;
  normalize deep-link URLs via assertHttpUrl.
- Tier 4 (cookies): confine the sign-in window's navigations/popups to web
  URLs (recursively) and deny all web permissions on its session.
- Tier 5 (deep-link/argv): bound the .url file read to 64 KB; match the
  aerofetch:// scheme case-insensitively.
- Tier 6 (Electron window): deny camera/mic/geolocation/USB/HID/serial/
  Bluetooth permissions on the app window.
- Tier 7 (network/persistence): restrict the watched-source RSS fetch to
  youtube.com feed URLs (SSRF guard); complete isValidSource validation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/sync.ts

@@ -7,11 +7,15 @@
 
 import { listSources, listMediaItems } from './sources'
 import { indexSource } from './indexer'
-import { parseRssVideoIds } from './indexerCore'
+import { parseRssVideoIds, isYouTubeFeedUrl } from './indexerCore'
 import type { IndexProgress, MediaItem, SyncResult } from '@shared/ipc'
 
 /** Fetch a YouTube RSS feed and return its recent video ids. Throws on failure. */
 async function fetchFeedIds(feedUrl: string): Promise<string[]> {
+  // Only ever fetch a genuine YouTube feed — refuse an arbitrary host that a
+  // corrupted sources.json might carry (SSRF guard, audit T7). A throw here is
+  // caught by the caller, which then falls back to a full yt-dlp re-index.
+  if (!isYouTubeFeedUrl(feedUrl)) throw new Error('Refusing to fetch a non-YouTube feed URL.')
   const res = await fetch(feedUrl, { signal: AbortSignal.timeout(15_000) })
   if (!res.ok) throw new Error(`feed responded ${res.status}`)
   return parseRssVideoIds(await res.text())