✨ 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>

src/config.ts

@@ -61,6 +61,9 @@ export const VILLAGER_NAMES = [
   'Orla','Pike','Quinn','Rook','Sera','Tull','Uma','Vex',
 ]
 
+/** Milliseconds after placement during which demolishing gives a full refund (linearly decays to 0%). */
+export const DEMOLISH_REFUND_MS = 180_000   // 3 minutes
+
 export const SAVE_KEY = 'tg_save_v5'
 export const AUTOSAVE_INTERVAL = 30_000