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/main/buildArgs.ts

@@ -104,6 +104,8 @@ export interface AccessOptions {
   rateLimit: string
   /** absolute path to aria2c.exe; omit to use yt-dlp's built-in downloader */
   aria2cPath?: string
+  /** aria2c parallel connections (-x/-s), clamped to 1–16; omitted = 16 (L64) */
+  aria2cConnections?: number
   /** read auth cookies from an installed browser's own cookie store */
   cookiesFromBrowser?: CookieBrowser
   /** absolute path to a Netscape-format cookie file (the sign-in window's export) */
@@ -118,9 +120,23 @@ export interface AccessOptions {
   youtubePoToken?: string
 }
 
-// Tuned for typical CDN-served video: 16 connections split across 16 segments,
-// each at least 1MB so tiny files don't get split pointlessly.
-export const ARIA2C_ARGS = 'aria2c:-x 16 -s 16 -k 1M'
+/**
+ * The yt-dlp `--downloader-args` value for aria2c: N connections split across N
+ * segments, each at least 1MB so tiny files don't get split pointlessly. The
+ * connection count is user-tunable (Settings → Network, L64) and clamped here to
+ * aria2c's own -x/-s ceiling of 16 — settings validation clamps too, but this is
+ * pure-module defence for any other caller. Non-finite input → the default 16.
+ */
+export function aria2cArgs(connections?: number): string {
+  const n =
+    connections !== undefined && Number.isFinite(connections)
+      ? Math.min(16, Math.max(1, Math.round(connections)))
+      : 16
+  return `aria2c:-x ${n} -s ${n} -k 1M`
+}
+
+/** The default tuning (16 connections) — kept for tests/docs that reference it. */
+export const ARIA2C_ARGS = aria2cArgs()
 
 /**
  * Shell-like split of a raw "extra yt-dlp args" string (Phase C custom-command
@@ -295,7 +311,9 @@ function accessArgs(a: AccessOptions): string[] {
   const args: string[] = []
   if (a.proxy.trim()) args.push('--proxy', a.proxy.trim())
   if (a.rateLimit.trim()) args.push('--limit-rate', a.rateLimit.trim())
-  if (a.aria2cPath) args.push('--downloader', a.aria2cPath, '--downloader-args', ARIA2C_ARGS)
+  if (a.aria2cPath) {
+    args.push('--downloader', a.aria2cPath, '--downloader-args', aria2cArgs(a.aria2cConnections))
+  }
   // cookiesFromBrowser and cookiesFile are mutually exclusive sources — the UI
   // only ever sets one (driven by Settings.cookieSource), but browser wins if
   // a caller somehow sets both.

src/main/download.ts

@@ -214,6 +214,7 @@ export function buildCommand(opts: StartDownloadOptions, cookiesFile?: string):
     proxy: settings.proxy,
     rateLimit: settings.rateLimit,
     aria2cPath,
+    aria2cConnections: settings.aria2cConnections,
     // L136/M6: incognito attaches no cookies from the browser either.
     cookiesFromBrowser:
       !opts.incognito && settings.cookieSource === 'browser' ? settings.cookiesBrowser : undefined,

src/main/ipc.ts

@@ -270,7 +270,10 @@ export function registerIpcHandlers(getMainWindow: () => BrowserWindow | null):
     })
   )
   ipcMain.handle(IpcChannels.scheduledSyncGet, () => getScheduledSync())
-  ipcMain.handle(IpcChannels.scheduledSyncSet, (_e, enabled: boolean) => setScheduledSync(enabled))
+  // `time` (24h HH:MM) is validated inside setScheduledSync before touching schtasks (L51).
+  ipcMain.handle(IpcChannels.scheduledSyncSet, (_e, enabled: boolean, time?: string) =>
+    setScheduledSync(enabled, time)
+  )
 
   // Reflect overall queue progress on the Windows taskbar and window title (SR8).
   ipcMain.handle(IpcChannels.taskbarProgress, (_e, p: TaskbarProgress) => {

src/main/schedule.ts

@@ -11,7 +11,7 @@
 
 import { execFile } from 'child_process'
 import { getSystem32Path } from './binaries'
-import type { ScheduledSyncStatus } from '@shared/ipc'
+import { isValidSyncTime, DEFAULT_SETTINGS, type ScheduledSyncStatus } from '@shared/ipc'
 
 const TASK_NAME = 'AeroFetchDailySync'
 /** The argv flag the scheduled task passes so startup knows it's a sync launch. */
@@ -46,10 +46,17 @@ export async function getScheduledSync(): Promise<ScheduledSyncStatus> {
 
 /**
  * Register or remove the daily-sync scheduled task. Creating runs AeroFetch with
- * `--sync` every day at 09:00 (overwriting any prior task of the same name).
+ * `--sync` every day at the given 24h 'HH:MM' time (overwriting any prior task of
+ * the same name — which is also how a time change re-registers, L51). The time is
+ * renderer-supplied over IPC, so it's validated here before reaching the schtasks
+ * argv; anything malformed falls back to the default rather than failing the toggle.
  */
-export async function setScheduledSync(enabled: boolean): Promise<ScheduledSyncStatus> {
+export async function setScheduledSync(
+  enabled: boolean,
+  time?: string
+): Promise<ScheduledSyncStatus> {
   if (enabled) {
+    const startTime = isValidSyncTime(time) ? time : DEFAULT_SETTINGS.syncTime
     const tr = `"${process.execPath}" ${SYNC_FLAG}`
     const r = await schtasks([
       '/Create',
@@ -57,7 +64,7 @@ export async function setScheduledSync(enabled: boolean): Promise<ScheduledSyncS
       '/SC',
       'DAILY',
       '/ST',
-      '09:00',
+      startTime,
       '/TN',
       TASK_NAME,
       '/TR',

src/main/settings.ts

@@ -14,6 +14,7 @@ import {
   DEFAULT_DOWNLOAD_OPTIONS,
   DEFAULT_SETTINGS,
   isYtdlpUpdateChannel,
+  isValidSyncTime,
   type Settings,
   type DownloadOptions,
   type SponsorBlockCategory,
@@ -266,6 +267,17 @@ function applySettings(s: Store<Settings>, partial: Partial<Settings>): void {
         if (Number.isFinite(n)) s.set('maxConcurrent', Math.min(5, Math.max(1, Math.round(n))))
         break
       }
+      case 'aria2cConnections': {
+        // aria2c caps -x/-s at 16; clamp like maxConcurrent rather than reject (L64).
+        const n = Number(value)
+        if (Number.isFinite(n)) s.set('aria2cConnections', Math.min(16, Math.max(1, Math.round(n))))
+        break
+      }
+      case 'syncTime':
+        // Reaches `schtasks /ST` when the daily sync is (re)registered — only a
+        // well-formed 24h HH:MM is ever stored (L51).
+        if (isValidSyncTime(value)) s.set('syncTime', value)
+        break
       case 'clipboardWatch':
       case 'useAria2c':
       case 'restrictFilenames':