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/binaries.ts

@@ -35,3 +35,15 @@ export function getFfprobePath(): string {
 export function getAria2cPath(): string {
   return join(getBinDir(), 'aria2c.exe')
 }
+
+/**
+ * Absolute path to a Windows system executable (e.g. taskkill.exe, schtasks.exe).
+ *
+ * SECURITY (audit F3): system tools are resolved by full path under System32
+ * rather than by bare name, so a same-named binary planted in the current
+ * working directory or earlier on PATH can't be invoked in their place — a real
+ * risk for the portable build, which runs from user-writable locations.
+ */
+export function getSystem32Path(exe: string): string {
+  return join(process.env.SystemRoot || 'C:\\Windows', 'System32', exe)
+}