ui: neutralize markdown preview colors for annotation clarity

Headings, bold, code all use muted gray tones instead of bright
cyan/blue so annotation highlights are clearly visible.

app/globals.css

@@ -677,14 +677,14 @@ h4 {
 }
 
 .markdown-preview {
-  color: #f1f5f9;
+  color: #cbd5e1;
 }
 
 .markdown-preview h1,
 .markdown-preview h2,
 .markdown-preview h3,
 .markdown-preview h4 {
-  color: #38bdf8;
+  color: #e2e8f0;
   margin-top: 1.2em;
   margin-bottom: 0.4em;
 }
@@ -705,7 +705,7 @@ h4 {
 
 .markdown-preview strong,
 .markdown-preview b {
-  color: #ffffff;
+  color: #e2e8f0;
   font-weight: 600;
 }
 
@@ -723,6 +723,7 @@ h4 {
   padding: 2px 4px;
   border-radius: 4px;
   font-size: 0.9em;
+  color: #cbd5e1;
 }
 
 .markdown-preview table {
@@ -740,6 +741,7 @@ h4 {
 
 .markdown-preview th {
   background-color: #334155;
+  color: #e2e8f0;
 }
 
 .markdown-content hr {