feat(service): landing chooser at /, shared header + footer (refs #451)

Post-merge, "/" was the admin console with a small text link to the
player. This makes "/" a neutral chooser and unifies the chrome across
all three surfaces (landing, player, admin).

- "/" now serves a lean chooser page (web/landing.html): a calm, self-
  contained page (no framework, inline CSS) that routes to the Player
  (/app) or the Admin & Setup console (/admin), with the console framed
  as the privileged surface. API/speaker clients (non-HTML Accept) still
  get the version JSON from "/" unchanged.
- The admin console moved to /admin (HandleAdmin); its assets and APIs
  are absolute, so it works unchanged at the new path.
- New persisted setting default_landing (chooser|app|admin): when set to
  app or admin, "/" 302-redirects straight there. Exposed in the admin
  Settings tab; defaults to the chooser.
- Shared header: all three carry the same accent bar (braille mark +
  "AfterTouch" + "Bose SoundTouch Toolkit"); the mark is the home link
  back to "/". Shared footer: all three show the same version line
  (the landing fetches /api/setup/version with a tiny vanilla script).

Light/dark and mobile refinements are deliberately left for a later
pass; the admin keeps its existing light-only styling for now.

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

cmd/soundtouch-service/main.go

@@ -1175,6 +1175,7 @@ func setupRouter(server *handlers.Server, stockholmHandler *stockholm.Handler, w
 	r.Use(server.RecordMiddleware)
 
 	r.Get("/", server.HandleRoot)
+	r.Get("/admin", server.HandleAdmin)
 	r.Get("/health", server.HandleHealth)
 	r.Get("/favicon.ico", func(w http.ResponseWriter, r *http.Request) {
 		// The favicon lives in the embedded web/img bundle, not under

cmd/soundtouch-service/testdata/router_routes.txt

@@ -34,6 +34,7 @@ GET      /accounts/{account}/devices/{device}/presets                 handlers.(
 GET      /accounts/{account}/devices/{device}/recents                 handlers.(*Server).HandleUnsupported-fm
 GET      /accounts/{account}/full                                     handlers.(*Server).HandleUnsupported-fm
 GET      /accounts/{account}/sources                                  handlers.(*Server).HandleUnsupported-fm
+GET      /admin                                                       handlers.(*Server).HandleAdmin-fm
 GET      /api/control/devices/                                        soundtouchweb.(*WebApp).HandleAPIDevices-fm
 GET      /api/control/devices/{id}/                                   soundtouchweb.(*WebApp).HandleAPIDevice-fm
 GET      /api/control/devices/{id}/action/{action}                    soundtouchweb.(*WebApp).HandleAPIControl-fm