v1.38.0 — Installable Web UI (PWA) + cross-channel version badge

Added

• Installable Web UI (PWA) — a web app manifest + icon so Docksentry can be added to a phone's home screen and run standalone, complementing the touch polish in v1.37.0 (#2). Served via the existing /static route; theme-color + apple-touch meta included. No new dependencies.
• Version badge on Discord / webhook / e-mail (#44, @LeeNX) — the 🔖 v_old → v_new info added to the Telegram "updates available" notification in v1.36.0 now also appears on the other channels: Discord embeds (per container), the generic webhook payload (old_version / new_version fields), and the e-mail summary. Empty when the image has no org.opencontainers.image.version label.

Full 19-script regression sweep green.