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/ytdlp.ts

@@ -1,7 +1,12 @@
 import { execFile } from 'child_process'
 import { existsSync } from 'fs'
 import { getYtdlpPath } from './binaries'
-import type { YtdlpVersionResult, YtdlpUpdateChannel, YtdlpUpdateResult } from '@shared/ipc'
+import {
+  isYtdlpUpdateChannel,
+  type YtdlpVersionResult,
+  type YtdlpUpdateChannel,
+  type YtdlpUpdateResult
+} from '@shared/ipc'
 
 /**
  * Step-1 spike: spawn the bundled yt-dlp and read back `--version`.
@@ -38,6 +43,14 @@ export function getYtdlpVersion(): Promise<YtdlpVersionResult> {
  * a per-user install; it would fail under a locked-down system install.
  */
 export function updateYtdlp(channel: YtdlpUpdateChannel): Promise<YtdlpUpdateResult> {
+  // Validate against the channel allowlist BEFORE the value reaches `--update-to`.
+  // That flag also accepts `OWNER/REPO@TAG`, which would download and install an
+  // arbitrary binary over yt-dlp.exe — so an unrecognised value (e.g. forged by a
+  // compromised renderer over IPC) must never be forwarded. (audit F1)
+  if (!isYtdlpUpdateChannel(channel)) {
+    return Promise.resolve({ ok: false, error: 'Unsupported update channel.' })
+  }
+
   const ytdlpPath = getYtdlpPath()
 
   if (!existsSync(ytdlpPath)) {