feat(audit): L51 + L64 — daily-sync time picker + aria2c connection tuning

Two small self-contained features from the audit's deferred list:

- L51: the Task Scheduler daily sync is no longer pinned to 09:00. New
  Settings.syncTime (24h HH:MM) with a native time input beside the Library
  "Daily sync" switch — persists on change, re-registers the task on blur
  (schtasks /Create /F overwrite is the time-change mechanism). The value
  reaches the schtasks /ST argv, so a shared isValidSyncTime guards it at
  both the settings write and in schedule.ts (unit-tested incl.
  injection-shaped input). Live-verified against the machine's real task.

- L64: aria2c connection count (-x/-s) is user-tunable. New
  Settings.aria2cConnections (1-16, aria2c's own ceiling) exposed as a
  SpinButton in Settings -> Network while the aria2c toggle is on.
  ARIA2C_ARGS became the pure aria2cArgs(n?) with a defensive clamp;
  threaded through AccessOptions. The -k 1M split floor stays fixed — a
  free-text field would bypass the custom-command consent gate.

typecheck + 309 tests + eslint + production build green.

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

src/shared/ipc.ts

@@ -319,6 +319,15 @@ export function isYtdlpUpdateChannel(v: unknown): v is YtdlpUpdateChannel {
   return typeof v === 'string' && (YTDLP_UPDATE_CHANNELS as readonly string[]).includes(v)
 }
 
+/**
+ * Runtime guard for a 24h 'HH:MM' sync time crossing the IPC boundary (L51) —
+ * the value reaches `schtasks /ST`, so anything malformed must never be forwarded.
+ * Shared so main's settings validation and schedule.ts use one definition.
+ */
+export function isValidSyncTime(v: unknown): v is string {
+  return typeof v === 'string' && /^([01]\d|2[0-3]):[0-5]\d$/.test(v)
+}
+
 export interface YtdlpUpdateResult {
   ok: boolean
   /** yt-dlp's own update output, e.g. "Updated to ... " or "yt-dlp is up to date" */
@@ -570,6 +579,8 @@ export interface Settings {
   rateLimit: string
   /** 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) */
+  aria2cConnections: number
   /** where auth cookies come from: none, a browser's store, or the sign-in window */
   cookieSource: CookieSource
   /** which browser to read cookies from when cookieSource is 'browser' */
@@ -603,6 +614,8 @@ export interface Settings {
   notifyOnComplete: boolean
   /** auto-enqueue newly-found videos from watched sources during a sync (Phase J) */
   autoDownloadNew: boolean
+  /** daily watched-source sync start time, 24h 'HH:MM' (Task Scheduler /ST) (L51) */
+  syncTime: string
   /** false until the first-run welcome screen has been dismissed */
   hasCompletedOnboarding: boolean
   /** keep AeroFetch running in the system tray when its window is closed (Phase O) */
@@ -650,6 +663,7 @@ export const DEFAULT_SETTINGS: Settings = {
   proxy: '',
   rateLimit: '',
   useAria2c: false,
+  aria2cConnections: 16,
   cookieSource: 'none',
   cookiesBrowser: 'chrome',
   youtubePlayerClient: '',
@@ -670,6 +684,7 @@ export const DEFAULT_SETTINGS: Settings = {
   // Default off so adding a watched channel doesn't silently fill the user's
   // disk on first use (SR3). Enable explicitly once they know what it does.
   autoDownloadNew: false,
+  syncTime: '09:00',
   hasCompletedOnboarding: false,
   minimizeToTray: false,
   launchAtStartup: false,