feat: self-managing yt-dlp + ffmpeg version display

yt-dlp is now a managed, auto-updating binary rather than a static bundled
one. On launch AeroFetch seeds a writable copy under userData from the
bundled seed, self-heals it if it goes missing, and — throttled to once a
day — self-updates it to the configured channel (default: nightly). This
keeps the binary current so YouTube changes don't silently cause 403s, and
an app reinstall (or portable re-extraction) can no longer roll it back.
Settings shows an auto-update toggle, the live version, last-checked time,
and the channel selector.

Also surface the bundled ffmpeg/ffprobe versions in Settings (display only:
they have no self-update path and, unlike yt-dlp, don't break as sites
change, so there is no auto-update for them).

- binaries: split the read-only bundled seed from the managed userData copy
- ytdlp: ensureManagedYtdlp (seed + self-heal), startup auto-update runner
- ytdlpPolicy: pure once-a-day throttle decision (unit-tested)
- ffmpeg: parse ffmpeg/ffprobe -version for the Settings panel
- settings/ipc/preload/renderer: new settings, IPC channels, types, and UI

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

test/ytdlpPolicy.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest'
+import { shouldAutoCheckYtdlp, AUTO_UPDATE_INTERVAL_MS } from '../src/main/ytdlpPolicy'
+
+const NOW = 1_782_435_469_486 // arbitrary fixed "now"
+
+describe('shouldAutoCheckYtdlp', () => {
+  it('never checks when auto-update is disabled', () => {
+    expect(shouldAutoCheckYtdlp(false, 0, NOW)).toBe(false)
+    // …even if a full interval has elapsed.
+    expect(shouldAutoCheckYtdlp(false, NOW - AUTO_UPDATE_INTERVAL_MS * 2, NOW)).toBe(false)
+  })
+
+  it('checks on first run (lastCheck is 0 / never)', () => {
+    expect(shouldAutoCheckYtdlp(true, 0, NOW)).toBe(true)
+  })
+
+  it('skips while inside the throttle window', () => {
+    // Checked one minute ago → far inside the 24h window.
+    expect(shouldAutoCheckYtdlp(true, NOW - 60_000, NOW)).toBe(false)
+    // Exactly one ms short of the interval still skips.
+    expect(shouldAutoCheckYtdlp(true, NOW - (AUTO_UPDATE_INTERVAL_MS - 1), NOW)).toBe(false)
+  })
+
+  it('checks once the interval has elapsed (boundary inclusive)', () => {
+    expect(shouldAutoCheckYtdlp(true, NOW - AUTO_UPDATE_INTERVAL_MS, NOW)).toBe(true)
+    expect(shouldAutoCheckYtdlp(true, NOW - AUTO_UPDATE_INTERVAL_MS * 3, NOW)).toBe(true)
+  })
+
+  it('honours a custom interval', () => {
+    const hour = 60 * 60 * 1000
+    expect(shouldAutoCheckYtdlp(true, NOW - 30 * 60_000, NOW, hour)).toBe(false)
+    expect(shouldAutoCheckYtdlp(true, NOW - hour, NOW, hour)).toBe(true)
+  })
+})