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>

.idea/gradle.xml

@@ -12,6 +12,7 @@
             <option value="$PROJECT_DIR$/modules" />
             <option value="$PROJECT_DIR$/modules/api" />
             <option value="$PROJECT_DIR$/modules/core" />
+            <option value="$PROJECT_DIR$/modules/ui" />
           </set>
         </option>
       </GradleProjectSettings>

.idea/scala_compiler.xml

@@ -5,7 +5,7 @@
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
     </profile>
-    <profile name="Gradle 2" modules="NowChessSystems.modules.core.main,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test">
+    <profile name="Gradle 2" modules="NowChessSystems.modules.core.main,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test,NowChessSystems.modules.ui.main,NowChessSystems.modules.ui.scoverage,NowChessSystems.modules.ui.test">
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
       <parameters>