Implement tile discarding, blocked doors, and correct corridor exits

- Updated TileDefinitions.js: Added 4-way exits to corridor_straight and corridor_steps (N/S y=3,4; E/W x=3,4).
- Updated DungeonGenerator.js: Added cancelPlacement() logic and onDoorBlocked callback.
- Updated GameRenderer.js: Implemented blockDoor() to visualize blocked passages, and improved isPlayerAdjacentToDoor.
- Updated UIManager.js: Added custom showModal/showConfirm and Discard button for tile placement.
- Updated main.js: Handled blocked door clicks and hooked up UI events.
- Updated GameEngine.js: Improved door adjacency checks.
- Updated CameraManager.js: Preserved camera rotation on centerOn.
- Added door1_blocked.png asset.

src/engine/dungeon/DungeonGenerator.js

@@ -27,6 +27,7 @@ export class DungeonGenerator {
         // Callbacks for UI
         this.onStateChange = null;
         this.onPlacementUpdate = null;
+        this.onDoorBlocked = null;
     }
 
     startDungeon(missionConfig) {
@@ -152,6 +153,30 @@ export class DungeonGenerator {
         return this.grid.canPlace(variant, this.placementX, this.placementY);
     }
 
+    cancelPlacement() {
+        if (this.state !== PLACEMENT_STATE.PLACING_TILE) return;
+
+        // 1. Mark door as blocked visually
+        if (this.onDoorBlocked && this.selectedExit) {
+            this.onDoorBlocked(this.selectedExit);
+        }
+
+        // 2. Remove the selected exit from available exits
+        if (this.selectedExit) {
+            this.availableExits = this.availableExits.filter(e =>
+                !(e.x === this.selectedExit.x && e.y === this.selectedExit.y && e.direction === this.selectedExit.direction)
+            );
+        }
+
+        // 3. Reset state
+        this.currentCard = null;
+        this.selectedExit = null;
+        this.state = PLACEMENT_STATE.WAITING_DOOR;
+
+        this.notifyPlacementUpdate();
+        this.notifyStateChange();
+    }
+
     /**
      * Confirm and finalize tile placement
      */