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/shared/ipc.ts

@@ -615,6 +615,20 @@ export interface Settings {
   proxy: string
   /** yt-dlp --limit-rate value, e.g. '2M' or '500K'. Empty means unlimited. */
   rateLimit: string
+  /**
+   * Request throttle (seconds), to avoid YouTube's "rate-limited for up to an
+   * hour" / HTTP 403 wall when pulling a whole channel. All default to 0 = off
+   * (yt-dlp's as-fast-as-possible behaviour):
+   *  - sleepRequests: `--sleep-requests`, a pause between the metadata/extraction
+   *    HTTP requests — the single most effective knob for large channel pulls.
+   *  - sleepInterval / maxSleepInterval: `--sleep-interval` (min) and
+   *    `--max-sleep-interval` (max) bracket a random pause before each download.
+   *    max only applies when it's set above a non-zero min (yt-dlp requires the
+   *    min to be present, and a max below it is meaningless).
+   */
+  sleepRequests: number
+  sleepInterval: number
+  maxSleepInterval: number
   /** use bundled aria2c (resources/bin/aria2c.exe) as the external downloader */
   useAria2c: boolean
   /** aria2c parallel connections per download (-x/-s), 1–16; only used when useAria2c is on (L64) */
@@ -709,6 +723,9 @@ export const DEFAULT_SETTINGS: Settings = {
   downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
   proxy: '',
   rateLimit: '',
+  sleepRequests: 0,
+  sleepInterval: 0,
+  maxSleepInterval: 0,
   useAria2c: false,
   aria2cConnections: 16,
   cookieSource: 'none',