auto-update wiring: silent download, restart banner, local-feed drill seam

electron-updater in the shell (packaged builds only; dev never updates).
Checks at startup + every 24h, downloads silently, and the renderer shows
a Restart banner once the update is staged (it installs on quit either
way). Manual "Check for Updates..." under the app menu answers via dialog.
Every updater event routes through the shell log and is caught - the
updater can never take the app down (verified live: a feed failure logs
three lines and the app runs on).

LH_UPDATE_FEED overrides the baked GitHub feed with a generic local
server - the pre-release drill: serve latest-mac.yml + zip + blockmap
from localhost and a real installed copy updates itself, no release, no
public repo.

The asar allow-list re-includes electron-updater + its 16-package runtime
tree explicitly (gitignore last-match-wins; asar verified at 2.1 MB).
The banner rides the preload bridge (onUpdateStatus/installUpdate) and
renders nothing in the browser. window.lh typing centralized in
sidebar.tsx's declaration.

package.json

@@ -37,6 +37,7 @@
 		"d3-shape": "^3.2.0",
 		"d3-zoom": "^3.0.0",
 		"dagre-d3-es": "7.0.14",
+		"electron-updater": "^6.8.9",
 		"embla-carousel-react": "^8.6.0",
 		"katex": "^0.16.47",
 		"linkedom": "^0.18.13",