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>

test/buildArgs.test.ts

@@ -9,6 +9,7 @@ import {
   collectionOutputTemplate,
   CROP_SQUARE_PPA,
   ARIA2C_ARGS,
+  aria2cArgs,
   DEST_MARKER,
   type AccessOptions
 } from '../src/main/buildArgs'
@@ -105,6 +106,25 @@ describe('network options', () => {
     const argv = build(opts(), dlo(), NO_ACCESS)
     expect(argv).not.toContain('--downloader-args')
   })
+
+  it('threads a custom aria2c connection count into --downloader-args (L64)', () => {
+    const argv = build(opts(), dlo(), {
+      ...NO_ACCESS,
+      aria2cPath: 'C:/fake/bin/aria2c.exe',
+      aria2cConnections: 4
+    })
+    expect(hasSeq(argv, '--downloader-args', 'aria2c:-x 4 -s 4 -k 1M')).toBe(true)
+  })
+
+  it('aria2cArgs clamps to aria2c’s 1–16 ceiling and defaults to 16 (L64)', () => {
+    expect(aria2cArgs()).toBe('aria2c:-x 16 -s 16 -k 1M')
+    expect(aria2cArgs(8)).toBe('aria2c:-x 8 -s 8 -k 1M')
+    expect(aria2cArgs(0)).toBe('aria2c:-x 1 -s 1 -k 1M') // below floor → 1
+    expect(aria2cArgs(99)).toBe('aria2c:-x 16 -s 16 -k 1M') // above ceiling → 16
+    expect(aria2cArgs(7.6)).toBe('aria2c:-x 8 -s 8 -k 1M') // rounds
+    expect(aria2cArgs(NaN)).toBe('aria2c:-x 16 -s 16 -k 1M') // non-finite → default
+    expect(ARIA2C_ARGS).toBe(aria2cArgs()) // the exported default stays in sync
+  })
 })
 
 // --- Cookies: browser store vs. sign-in window's exported file --------------