Fixes #376: End-turn buttons double-advancing turns in Observer Mode

[Billytifft]

What was wrong

In Observer Mode, pressing Enter would often advance the turn twice.
Enter (and Space) are bound to the end_turn input action, which calls
OnPlayerEndTurn. But the two end-turn TextureButtons in the lower-right
info box were keyboard-focusable, so when either had focus Godot's
ui_accept fired them too, emitting BlinkyEndTurnButtonPressed ->
OnPlayerEndTurn a second time. NumPad Enter advanced only once because
it is not bound to end_turn, matching the reported behavior.

How it was fixed

Set FocusMode = FocusModeEnum.None on nextTurnButton and
boxRightRectangleButton in LowerRightInfoBox.cs, so Enter/Space no
longer activate them via ui_accept. Mouse clicks still work. This uses
the same pattern already established in ConsoleButton.cs.

How it was tested

Verified manually in Godot with Observer Mode (Cmd+Shift+Alt+O, fast
forward set to 0):

• Click the end-turn button once, then press Enter repeatedly - exactly
  one turn per press (was two before the fix).
• Same result with Space and NumPad Enter.
• Clicking the invisible box area (boxRightRectangleButton) then Enter
  advances exactly one turn.
• Normal (non-observer) play regression: end-turn button clicks still
  work and arrow-key unit movement is unaffected.

Build: 0 errors. dotnet format ... --verify-no-changes: clean.
Tests: 74/74 passed.

Fixes #376

[ajhalme]

LGTM