v0.8.0 — Update-available detection (version chip + changelog)

Footer becomes a changelog chip and shows an 'Update available' badge when a
newer APP_VERSION exists upstream. Detection reads src/lib/version.ts from the
Gitea default branch (dms model) and compares — self-contained, no OTM control
plane. Adds src/lib/update-check.ts and GET /api/version (also a running-version
probe for the upcoming one-click rebuild).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/version.ts

@@ -1,2 +1,2 @@
 // Bump on every user-visible release. Changelog entries live in `src/lib/changelog.ts`.
-export const APP_VERSION = "0.7.0";
+export const APP_VERSION = "0.8.0";