feat: add offline progress calculation (closes #11)

Saves lastSaveTime timestamp. On load, calculates elapsed time
and awards 50% efficiency production. Shows summary toast.
Min 30 seconds away to trigger.

js/render.js

@@ -206,6 +206,7 @@ function saveGame() {
         totalEventsResolved: G.totalEventsResolved || 0,
         buyAmount: G.buyAmount || 1,
         playTime: G.playTime || 0,
+        lastSaveTime: Date.now(),
         sprintActive: G.sprintActive || false,
         sprintTimer: G.sprintTimer || 0,
         sprintCooldown: G.sprintCooldown || 0,