✨ add demolish mode for buildings (Issue #50)

- New 💥 Demolish button in the action bar
- Red ghost highlights building footprint on hover
- Refund: 100% within 3 min, decays linearly to 0%
- Mine teardown unblocks passability tiles and removes status label
- Nisse inside demolished mine are rescued and reset to idle
- Floor/wall/chest tiles restored to GRASS on demolish
- Build error now shows missing resources instead of generic message
- BuildingState gains builtAt field; old saves default to 0 (no refund)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## [Unreleased]
 
 ### Added
+- **Demolish Mode** (Issue #50): New 💥 Demolish button in the action bar; hover shows a red ghost over any building with a refund percentage; buildings demolished within 3 minutes return 100% of costs (linear decay to 0%); mine footprint tiles are unblocked on teardown; Nisse working inside a demolished building are rescued and resume idle; tile types are restored where applicable (floor/wall/chest → grass)
 - **Mine Building** (Issue #42): 3×2 building placeable only on resource-free ROCK tiles (costs: 200 wood + 50 stone); Nisse with mine priority walk to the entrance, disappear inside for 15 s, then reappear carrying 2 stone; up to 3 Nisse work simultaneously; ⛏ X/3 status label shown directly on the building in world space; surface rock harvesting remains functional alongside the building
 
 ### Fixed