Fix session title input's border offset and width

The -mx-2 trick pushed the hover/focus border noticeably left of the
text, and flex-1 stretched the input across the whole header row.
Tighten the offset to px-1.5/-ml-1.5 and size the input to its content
via field-sizing:content instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WzyfRyA7h5rs5SCAahLAWd

components/sessions/SessionTitleEditor.tsx

@@ -46,13 +46,13 @@ export function SessionTitleEditor({ sessionId, initialName }: { sessionId: numb
   }, [status]);
 
   return (
-    <div className="-mx-2 flex items-center gap-2">
+    <div className="-ml-1.5 flex items-center gap-2">
       <input
         value={value}
         onChange={(e) => setValue(e.target.value)}
         placeholder={`Session #${sessionId}`}
         aria-label="Session name"
-        className="min-w-0 flex-1 rounded-md border border-transparent bg-transparent px-2 py-0.5 font-heading text-2xl font-semibold text-foreground outline-none transition-colors placeholder:text-muted-foreground/70 hover:border-border focus:border-border focus:bg-muted/30"
+        className="min-w-0 max-w-full rounded-md border border-transparent bg-transparent px-1.5 py-0.5 font-heading text-2xl font-semibold text-foreground outline-none transition-colors placeholder:text-muted-foreground/70 hover:border-border focus:border-border focus:bg-muted/30 [field-sizing:content]"
       />
       <div className="relative flex size-4 shrink-0 items-center justify-center">
         <Loader2