Background running for downloads/auto-download, library clipboard detect, updater auth + token

Closing the window no longer kills in-progress downloads, the library gains the
same copied-link suggestion the downloads tab has, and the in-app updater can now
authenticate to a sign-in-required Gitea.

Background / tray:
- The window's close handler now hides to the tray (instead of quitting) whenever
  a download is in flight, even if "Keep running in the tray" is off — quitting
  was killing the spawned yt-dlp processes. A one-time notification explains the
  app is still running. (download.ts exposes hasActiveDownloads().)
- tray.ts now falls back to an embedded icon when no build/icon.ico ships, so the
  tray actually appears — previously an empty icon meant the tray was skipped and
  a minimized window could be stranded with no way back.
- New "Start with Windows" setting (launchAtStartup) wired to
  app.setLoginItemSettings, synced at startup and on toggle. Useful with
  auto-download so watched channels stay current in the background.
- The "Keep running in the tray" hint now explains it also enables background
  auto-download of new uploads.

Library clipboard detection:
- Extracted the downloads tab's clipboard watcher into a shared useClipboardLink
  hook and used it in the library's add-source field, so a copied channel/playlist
  link is offered there too.

Updater fix (works on a private / sign-in-required instance):
- Added an optional updateToken setting. When set, the updater sends it as a Gitea
  Authorization header on the release check, the checksum fetch, and the installer
  download — so "Check for updates" works where anonymous access is blocked (the
  previous "could not reach the update server" case). Blank = anonymous, unchanged.
  No token is ever shipped; it's only ever sent to the host-pinned update host.
  Settings gains a masked "Update access token" field.

Typecheck clean; 187 tests pass; electron-vite build clean. New UI verified in the
browser preview (tray/startup toggles, token field, library copied-link banner).

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

src/main/download.ts

@@ -43,6 +43,15 @@ interface ActiveDownload {
 
 const active = new Map<string, ActiveDownload>()
 
+/**
+ * Whether any yt-dlp download is currently running. Used by the window's close
+ * handler to keep the app alive in the tray (instead of quitting and killing the
+ * spawned processes) when the user closes the window mid-download.
+ */
+export function hasActiveDownloads(): boolean {
+  return active.size > 0
+}
+
 // --- Formatting helpers (raw yt-dlp numbers → human strings) ----------------
 
 function num(s?: string): number | undefined {