Add player setup form with validation and field error highlighting

- Dynamic name fields (3–6 players) with add/remove controls
- Start button disabled until all names are non-blank and unique;
  uniqueness check normalizes internal whitespace so "Bob  Smith"
  and "Bob Smith" are treated as the same name
- Names passed to game state are edge-trimmed only, preserving
  internal spacing as the user typed it
- Failing fields highlighted with red border and light red background
- PrimaryButton gains disabled prop with faded/flat appearance;
  active press state suppressed when disabled
- All round buttons now use consistent shadow and translateY values

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/PrimaryButton.module.css

@@ -11,7 +11,13 @@
   transition: transform 80ms ease, box-shadow 80ms ease;
 }
 
-.button:active {
+.button:active:not(:disabled) {
   transform: translateY(2px);
   box-shadow: 0 2px 6px rgba(0, 0, 0, 0.2);
 }
+
+.button:disabled {
+  opacity: 0.45;
+  box-shadow: none;
+  cursor: not-allowed;
+}