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/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) => {