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/preload/index.ts

@@ -218,11 +218,12 @@ const api = {
    *  match the main function `syncWatchedSources` (L92). */
   syncWatchedSources: (): Promise<SyncResult> => ipcRenderer.invoke(IpcChannels.sourcesSync),
 
-  /** Read / write the Windows daily-sync scheduled task. */
+  /** Read / write the Windows daily-sync scheduled task. `time` is the 24h 'HH:MM'
+   *  start time (L51); omitted = the default. Re-creating with a new time updates it. */
   getScheduledSync: (): Promise<ScheduledSyncStatus> =>
     ipcRenderer.invoke(IpcChannels.scheduledSyncGet),
-  setScheduledSync: (enabled: boolean): Promise<ScheduledSyncStatus> =>
-    ipcRenderer.invoke(IpcChannels.scheduledSyncSet, enabled),
+  setScheduledSync: (enabled: boolean, time?: string): Promise<ScheduledSyncStatus> =>
+    ipcRenderer.invoke(IpcChannels.scheduledSyncSet, enabled, time),
 
   /** Subscribe to live indexing progress. Returns an unsubscribe function. */
   onIndexProgress: (cb: (p: IndexProgress) => void): (() => void) => {