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/tray.ts

@@ -7,6 +7,16 @@
 import { app, Tray, Menu, nativeImage, type BrowserWindow } from 'electron'
 import { getAppIconPath } from './binaries'
 
+// Fallback tray glyph (32×32 teal disc + white download arrow) used when no
+// build/icon.ico is present. Without this the tray is skipped (an empty image
+// makes an invisible/unclickable Windows tray entry), which would strand a
+// minimized-to-tray window with no way back. Embedded as base64 so it needs no
+// asset-bundling step.
+const FALLBACK_TRAY_PNG =
+  'iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAAl0lEQVR4nO3TQQ6AIAxEUU7giV17bd0i' +
+  'aaDTmdqY0ISl/peirf11jvO6+/N5cHXKwlIIG6cQqngIoY5DiKy4C4G8aBwJohSArpIBmIgNKAWgDys' +
+  'AL8QGlANmCHZc8dUW1HEYEEFA6/d+B6q4CVAhwnHkb2DiUwCDkMRRBHpccQsRnXB8RLCAULxHMAAqbm0j5b4VoPRg1jzWxpzrS/JA7QAAAABJRU5ErkJggg=='
+
 let tray: Tray | null = null
 // True once the user has chosen to really quit (tray menu, or app.quit from the
 // updater) — lets the window 'close' handler tell "hide to tray" from "exit".
@@ -31,9 +41,10 @@ function show(getWindow: () => BrowserWindow | null): void {
 /** Create the tray icon + menu once. No-op if it already exists or the icon is missing. */
 export function createTray(getWindow: () => BrowserWindow | null): void {
   if (tray) return
-  const icon = nativeImage.createFromPath(getAppIconPath())
-  // Without a usable icon a Windows tray entry is invisible/unclickable, which is
-  // worse than no tray — so skip it rather than ship a dead tray.
+  // Prefer the real app icon; fall back to the embedded glyph when no icon.ico
+  // ships, so minimize-to-tray always has a tray to restore from.
+  let icon = nativeImage.createFromPath(getAppIconPath())
+  if (icon.isEmpty()) icon = nativeImage.createFromDataURL(`data:image/png;base64,${FALLBACK_TRAY_PNG}`)
   if (icon.isEmpty()) return
 
   tray = new Tray(icon)