feat(city): add east shore beach leisure activity

Characters can now relax on the beach sand to recover energy and
health. The server clamps position to the shoreline, exposes beach
geometry in the city payload, and the client renders animated waves,
sand, stalls, chairs, and ambient NPCs.

Includes Beach support class, new beach action endpoint, localization
strings, and feature tests.

lang/es.json

@@ -212,6 +212,13 @@
     "city.exercise_active": "Ejercitándose",
     "city.exercise_stopHint": "para parar",
     "city.exercise_ok": "¡Bien! +2 de salud.",
+    "city.beach_need": "Para relajar, ve a la playa.",
+    "city.beach_ok": "¡Relajaste en la playa y recuperaste energía!",
+    "city.beach_full": "La energía y la salud ya están al máximo.",
+    "city.beach_busy": "Termina el ejercicio actual primero.",
+    "city.beach_start": "Descansando en la arena…",
+    "city.beach_prompt": "Relajar en la playa",
+    "city.beach_active": "Relajándote en la playa",
     "city.exercise_need_equipment": "Acérquese a un equipo de la plaza.",
     "city.exercise_need_awake": "Debe estar despierto para ejercitarse.",
     "city.exercise_tired": "Está demasiado cansado para ejercitarse.",