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>

resources/bin/README.md

@@ -4,12 +4,14 @@ AeroFetch spawns these from the **main process** and bundles them via
 electron-builder `extraResources` (they ship outside the asar archive). They are
 **not** committed to git (see `.gitignore`).
 
-Drop the two Windows executables here before running `npm run dev` or building:
+Drop the two required Windows executables here before running `npm run dev` or
+building; `aria2c.exe` is optional:
 
 ```
 resources/bin/
 ├── yt-dlp.exe
-└── ffmpeg.exe
+├── ffmpeg.exe
+└── aria2c.exe   (optional)
 ```
 
 ## yt-dlp.exe
@@ -26,5 +28,14 @@ resources/bin/
 - Copy `ffmpeg.exe` here. Ship the LGPL license text alongside the installer
   before release.
 
+## aria2c.exe (optional)
+
+- Powers the "Use aria2c downloader" toggle in Settings → Network for faster
+  multi-connection downloads. If it's missing, that toggle silently falls back
+  to yt-dlp's built-in downloader — nothing breaks.
+- Download: https://github.com/aria2/aria2/releases (grab the `aria2-*-win-64bit-build1.zip`,
+  copy `aria2c.exe` from it here).
+- GPL-2.0 — ship its license text alongside the installer before release if bundled.
+
 > Branding stays generic: AeroFetch is a "yt-dlp frontend." Keep upstream
 > license texts with any distributed build.