Add time limit modifier for public games ⏱️

Adds a "time limit" modifier (10/20/30 min) to the public special-game
modifier pool, alongside existing modifiers like starting gold and water
nukes. Reuses the existing maxTimerValue config field, which WinCheckExecution
already enforces (territory leader wins when the timer expires).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

resources/lang/en.json

@@ -1128,6 +1128,8 @@
     "sams_disabled_label": "SAMs",
     "starting_gold": "{amount}M Starting Gold",
     "starting_gold_label": "Starting Gold",
+    "time_limit": "{amount}min Time Limit",
+    "time_limit_label": "Time Limit",
     "water_nukes": "Water Nukes",
     "water_nukes_label": "Water Nukes"
   },