fix(audiobookshelf): mount the app at the base path its SPA was built for

ROUTER_BASE_PATH=/ served a healthy API and a web UI that could never boot. The index.html in the pinned image is built with routerBasePath:/audiobookshelf, so every script request fell through to the SPA fallback and came back as text/html, which Chrome refuses to execute. healthcheck, the API and the phone app all stayed fine, which is exactly how this hid behind an API-only deploy check.

The deployer now reads the mounted base path out of the unit and hands it to the bootstrapper as ABS_URL, so the API door, the login route and the OIDC redirect subfolder cannot drift from the unit. healthcheck answers at the root under either base path, so the health gate is unchanged.

tests/dankserver_quadlet.bats

@@ -182,8 +182,13 @@ load helpers
     # under base path /audiobookshelf and the deploy gate timed out beside a unit
     # that was active (running). PORT and ROUTER_BASE_PATH are what the image's own
     # ENV block and the server source actually use.
+    # The base path is not a free choice either: /app/client/dist/index.html inside the
+    # pinned image carries routerBasePath:"/audiobookshelf", so a server mounted at / hands
+    # out a page whose every script request falls through to the SPA fallback and comes back
+    # text/html — the UI never leaves "Loading…" while the API stays healthy (measured
+    # 2026-11-02, which is exactly how it hid behind an API-only verification).
     unit="$REPO_ROOT/dankserver/audiobookshelf/quadlet/audiobookshelf.container"
     grep -q '^Environment=PORT=8000$' "$unit" || return 1
-    grep -q '^Environment=ROUTER_BASE_PATH=/$' "$unit" || return 1
+    grep -q '^Environment=ROUTER_BASE_PATH=/audiobookshelf$' "$unit" || return 1
     ! grep -qE '^Environment=ABS_' "$unit" || return 1
 }