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/view/CameraManager.js

@@ -52,9 +52,14 @@ export class CameraManager {
     }
 
     centerOn(x, y) {
-        // Grid (x, y) -> World (x, 0, -y)
+        // Calculate current offset relative to OLD target
+        const currentOffset = this.camera.position.clone().sub(this.target);
+
+        // Update target: Grid (x, y) -> World (x, 0, -y)
         this.target.set(x, 0, -y);
-        this.camera.position.copy(this.target).add(this.isoOffset);
+
+        // Restore position with new target + same relative offset
+        this.camera.position.copy(this.target).add(currentOffset);
         this.camera.lookAt(this.target);
     }