fix(ui): correct dice roller 'Save Roll' button positioning

The Save Roll button was appearing at the bottom left of the container
instead of being centered below the result.

Added flexbox layout to .rpg-dice-result container:
- display: flex
- flex-direction: column
- align-items: center

This ensures the button is properly centered and positioned below
the dice result value and details.

style.css

@@ -2395,6 +2395,9 @@ body:has(.rpg-panel.rpg-position-left) #sheld {
 
 /* Result Section */
 .rpg-dice-result {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
     text-align: center;
     padding: var(--modal-padding);
     background: rgba(0, 0, 0, 0.3);