fix: Replace Instrument Serif with Playfair Display for display headings

Instrument Serif is inherently condensed and illegible at heading sizes.
Playfair Display is wider, has proper variable weight support (400-900),
and reads well at all sizes.

apps/web/src/components/InboxList.tsx

@@ -122,7 +122,7 @@ export function InboxList({
       {/* Header */}
       <div className="flex items-center justify-between">
         <div className="flex items-center gap-2">
-          <h2 className="text-2xl font-display">Agent Inbox</h2>
+          <h2 className="text-lg font-semibold font-display">Agent Inbox</h2>
           <Badge variant="secondary">{joined.length}</Badge>
         </div>
         <Button variant="outline" size="sm" onClick={onRefresh}>

apps/web/src/components/PhaseAccordion.tsx

@@ -60,7 +60,7 @@ export function PhaseAccordion({
         )}
 
         {/* Phase name */}
-        <span className="min-w-0 flex-1 truncate font-display">
+        <span className="min-w-0 flex-1 truncate font-display font-medium">
           {phase.name}
         </span>