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/deeplink.ts

@@ -1,27 +1,40 @@
 import { app, shell, type BrowserWindow } from 'electron'
-import { existsSync, readFileSync } from 'fs'
+import { existsSync, openSync, readSync, closeSync } from 'fs'
 import { join } from 'path'
+import { assertHttpUrl } from './url'
 
 /** Only ever forward http(s) targets into the app — same restriction the
- *  external-link window-open handler in index.ts applies to in-page links. */
+ *  external-link window-open handler in index.ts applies to in-page links.
+ *  Delegates to the download path's guard so an incoming deep-link target is
+ *  validated AND parser-normalised (no embedded tabs/newlines/control chars
+ *  reach the renderer); returns null instead of throwing for the argv scan.
+ *  (audit T3 / F5) */
 function asHttpUrl(candidate: string): string | null {
   try {
-    const u = new URL(candidate)
-    return u.protocol === 'http:' || u.protocol === 'https:' ? candidate : null
+    return assertHttpUrl(candidate)
   } catch {
     return null
   }
 }
 
 /** Reads a Windows Internet Shortcut (.url) file's target — what Explorer's
- *  "Send to" menu hands us when the user sends a saved link to AeroFetch. */
+ *  "Send to" menu hands us when the user sends a saved link to AeroFetch.
+ *  Only the first 64 KB is read: a real shortcut is a few hundred bytes, so this
+ *  caps memory and limits exposure if argv points at a pathological or oversized
+ *  file that merely ends in `.url`. (audit T5) */
+const MAX_URL_FILE_BYTES = 64 * 1024
 function readUrlShortcut(path: string): string | null {
+  let fd: number | null = null
   try {
-    const text = readFileSync(path, 'utf8')
-    const match = /^URL=(.+)$/im.exec(text)
+    fd = openSync(path, 'r')
+    const buf = Buffer.alloc(MAX_URL_FILE_BYTES)
+    const bytes = readSync(fd, buf, 0, buf.length, 0)
+    const match = /^URL=(.+)$/im.exec(buf.toString('utf8', 0, bytes))
     return match ? asHttpUrl(match[1].trim()) : null
   } catch {
     return null
+  } finally {
+    if (fd !== null) closeSync(fd)
   }
 }
 
@@ -32,7 +45,7 @@ function readUrlShortcut(path: string): string | null {
  */
 export function extractIncomingUrl(argv: string[]): string | null {
   for (const arg of argv) {
-    if (arg.startsWith('aerofetch://')) {
+    if (/^aerofetch:\/\//i.test(arg)) {
       try {
         const target = new URL(arg).searchParams.get('url')
         const valid = target && asHttpUrl(target)