fix: align diagram action buttons to var(--sans) font, consistent 12px size

diagram-retry-btn + diagram-ask-btn: mono → sans, removed letter-spacing

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

ui/src/index.css

@@ -2224,8 +2224,8 @@ textarea:focus-visible {
 }
 
 .diagram-ask-btn {
-  font-size: 11px;
-  font-family: var(--mono);
+  font-size: 12px;
+  font-family: var(--sans);
   font-weight: 600;
   color: #fff;
   background: linear-gradient(135deg, var(--accent) 0%, #5B8FF9 100%);
@@ -2233,7 +2233,6 @@ textarea:focus-visible {
   border-radius: 6px;
   padding: 5px 12px;
   cursor: pointer;
-  letter-spacing: 0.02em;
   box-shadow: 0 1px 6px rgba(91, 143, 249,0.25), inset 0 1px 0 rgba(255,255,255,0.10);
   transition: background var(--transition), box-shadow var(--transition), transform 80ms ease;
 }
@@ -2360,8 +2359,8 @@ textarea:focus-visible {
 }
 
 .diagram-retry-btn {
-  font-size: 11px;
-  font-family: var(--mono);
+  font-size: 12px;
+  font-family: var(--sans);
   font-weight: 500;
   color: var(--text-2);
   background: var(--surface-3);
@@ -2369,7 +2368,6 @@ textarea:focus-visible {
   border-radius: 6px;
   padding: 5px 12px;
   cursor: pointer;
-  letter-spacing: 0.02em;
   transition: color var(--transition), background var(--transition), border-color var(--transition);
 }
 .diagram-retry-btn:hover {