Add Phase B: cookies, aria2c, proxy/rate-limit, restrict-filenames & archive

Implements the remaining Phase B (Access & networking) items from
ROADMAP.md: cookie sources (browser cookie-store import, or a built-in
sign-in window that exports a Netscape cookie file via a persisted Electron
session partition), the aria2c external downloader, proxy/rate-limit,
restrict-filenames, and a download-archive to skip repeats.

Wires download.ts to the buildArgs() module added in the previous commit
(it wasn't hooked up yet) and renames its options param
NetworkOptions -> AccessOptions to reflect the wider scope now that cookies
and filename/archive flags joined proxy/rate-limit/aria2c.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/binaries.ts

@@ -20,3 +20,8 @@ export function getYtdlpPath(): string {
 export function getFfmpegPath(): string {
   return join(getBinDir(), 'ffmpeg.exe')
 }
+
+/** Optional bundled external downloader; absent unless dropped into resources/bin. */
+export function getAria2cPath(): string {
+  return join(getBinDir(), 'aria2c.exe')
+}