feat(network): request-throttle controls to avoid YouTube rate-limiting

Re-implements the sleepRequests/sleepInterval/maxSleepInterval feature
(--sleep-requests / --sleep-interval / --max-sleep-interval) from the
stale feat/binary-management-and-library-scale branch, ported onto
current main's structure (zod settingsSchema, decomposed NetworkCard).
All default to 0/off, preserving current behaviour.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/download.ts

@@ -222,6 +222,9 @@ export function buildCommand(opts: StartDownloadOptions, cookiesFile?: string):
   const access = {
     proxy: settings.proxy,
     rateLimit: settings.rateLimit,
+    sleepRequests: settings.sleepRequests,
+    sleepInterval: settings.sleepInterval,
+    maxSleepInterval: settings.maxSleepInterval,
     aria2cPath,
     aria2cConnections: settings.aria2cConnections,
     // L136/M6: incognito attaches no cookies from the browser either.