fix: improve TerminalUI coverage and make MoveCommand/ResetCommand immutable

**TerminalUI Coverage Fix:**
- Added explicit test for empty input case (lines 64-65 previously uncovered)
- Test "TerminalUI should explicitly handle empty input by re-prompting" validates
  that multiple empty inputs are properly handled by re-prompting
- UI module coverage improved to near-100%

**MoveCommand Immutability (Anti-pattern Fix):**
- Changed MoveCommand fields from var to val: moveResult, previousBoard,
  previousHistory, previousTurn
- Changed ResetCommand fields from var to val: previousBoard, previousHistory,
  previousTurn
- Updated GameEngine to use .copy() instead of direct mutation when updating
  command state (lines 88, 95, 103, 112)
- Removed 3 edge-case tests that relied on command mutation (now impossible with
  immutable fields)

**MoveCommand Immutability Tests:**
- Added MoveCommandImmutabilityTest to verify:
  - Fields cannot be mutated after creation
  - equals/hashCode respect immutability invariant
  - .copy() creates new instances with updated values

All tests pass; 100% core coverage maintained.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala

@@ -46,6 +46,30 @@ class TerminalUITest extends AnyFunSuite with Matchers {
     output.split("White's turn.").length should be > 2
   }
 
+  test("TerminalUI should explicitly handle empty input by re-prompting") {
+    val in = new ByteArrayInputStream("\n\nq\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    val output = out.toString
+    // With two empty inputs, prompt should appear at least 4 times:
+    // 1. Initial board display
+    // 2. After first empty input
+    // 3. After second empty input
+    // 4. Before quit
+    val promptCount = output.split("White's turn.").length
+    promptCount should be >= 4
+    output should include("Game over. Goodbye!")
+  }
+
   test("TerminalUI printPrompt should include undo and redo hints if engine returns true") {
     val in = new ByteArrayInputStream("\nq\n".getBytes)
     val out = new ByteArrayOutputStream()