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/indexer.ts

@@ -83,8 +83,9 @@ export async function indexSource(
   url: string,
   onProgress: (p: IndexProgress) => void
 ): Promise<IndexSourceResult> {
+  let normalizedUrl: string
   try {
-    assertHttpUrl(url)
+    normalizedUrl = assertHttpUrl(url) // normalised form for spawning (audit F5)
   } catch (e) {
     return { ok: false, error: (e as Error).message }
   }
@@ -146,7 +147,7 @@ export async function indexSource(
       // resolve to a playlist when probed. A lone video has no entries → error.
       kind = cls?.kind ?? 'playlist'
       onProgress({ url, phase: 'uploads', message: 'Indexing playlist…' })
-      const data = await probeFlat(cls?.base ?? url)
+      const data = await probeFlat(cls?.base ?? normalizedUrl)
       const entries = data.entries ?? []
       if (entries.length === 0) {
         return {