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/en.json

@@ -212,6 +212,13 @@
     "city.exercise_active": "Exercising",
     "city.exercise_stopHint": "to stop",
     "city.exercise_ok": "Nice! +2 health.",
+    "city.beach_need": "To relax, head to the beach.",
+    "city.beach_ok": "You relaxed at the beach and recovered energy!",
+    "city.beach_full": "Energy and health are already at max.",
+    "city.beach_busy": "Finish your current exercise first.",
+    "city.beach_start": "Resting on the sand…",
+    "city.beach_prompt": "Relax at the beach",
+    "city.beach_active": "Relaxing at the beach",
     "city.exercise_need_equipment": "Get closer to plaza gym equipment.",
     "city.exercise_need_awake": "You must be awake to exercise.",
     "city.exercise_tired": "You are too tired to exercise.",