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/url.ts

@@ -10,17 +10,23 @@
  * (Call sites also pass `--` before the positional URL as defence in depth.)
  *
  * Throws a user-friendly Error on anything that isn't an http(s) URL.
+ *
+ * Returns the parser-NORMALISED URL (`u.href`), not the raw input (audit F5).
+ * The WHATWG URL parser silently tolerates interior tab/newline characters and
+ * leading C0 control bytes (which `String.trim()` does not strip), so returning
+ * the raw string could hand a downstream consumer — argv, or the sign-in
+ * window's loadURL — a value subtly different from the one actually validated.
+ * Emitting `u.href` guarantees callers use exactly the URL that passed the check.
  */
 export function assertHttpUrl(raw: string): string {
-  const trimmed = (raw ?? '').trim()
   let u: URL
   try {
-    u = new URL(trimmed)
+    u = new URL((raw ?? '').trim())
   } catch {
     throw new Error('That doesn’t look like a valid URL.')
   }
   if (u.protocol !== 'http:' && u.protocol !== 'https:') {
     throw new Error('Only http and https links are supported.')
   }
-  return trimmed
+  return u.href
 }