Make the session description always visible and inline-editable

New SessionDescriptionEditor renders even when empty (with an "Add a
description..." placeholder) and autosaves on typing - same debounced,
no-Save-button pattern as the title and device name editors. Unlike
the name field, an empty description is a valid saved state.

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

app/(app)/sessions/[id]/page.tsx

@@ -6,6 +6,7 @@ import { Button } from "@/components/ui/button";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { SessionTimelineChart } from "@/components/sessions/SessionTimelineChart";
 import { SessionTitleEditor } from "@/components/sessions/SessionTitleEditor";
+import { SessionDescriptionEditor } from "@/components/sessions/SessionDescriptionEditor";
 import { STATUS_VARIANT } from "@/components/sessions/SessionsTable";
 import { getSessionDetail, getSessionName } from "@/lib/db/queries/sessions";
 import { getSessionTimeline } from "@/lib/db/queries/stats";
@@ -52,9 +53,7 @@ export default async function SessionDetailPage({ params }: { params: Promise<{
               )}
             </p>
           )}
-          {detail.session.description && (
-            <p className="mt-1 text-sm text-muted-foreground">{detail.session.description}</p>
-          )}
+          <SessionDescriptionEditor sessionId={detail.session.id} initialDescription={detail.session.description} />
         </div>
         <div className="flex shrink-0 items-center gap-3">
           <Badge variant={STATUS_VARIANT[detail.session.status]}>{detail.session.status}</Badge>

components/sessions/SessionDescriptionEditor.tsx

@@ -0,0 +1,81 @@
+"use client";
+
+import { useEffect, useRef, useState } from "react";
+import { useRouter } from "next/navigation";
+import { Check, Loader2 } from "lucide-react";
+import { toast } from "sonner";
+import { cn } from "@/lib/utils";
+
+const AUTOSAVE_DELAY_MS = 600;
+
+export function SessionDescriptionEditor({
+  sessionId,
+  initialDescription,
+}: {
+  sessionId: number;
+  initialDescription: string | null;
+}) {
+  const router = useRouter();
+  const initial = initialDescription ?? "";
+  const [value, setValue] = useState(initial);
+  const [status, setStatus] = useState<"idle" | "saving" | "saved">("idle");
+  const lastSaved = useRef(initial);
+
+  useEffect(() => {
+    // Unlike the session name, an empty description is a valid, saveable
+    // state (it just means "no description"), so no non-empty guard here.
+    if (value === lastSaved.current) return;
+
+    setStatus("saving");
+    const timer = setTimeout(async () => {
+      const res = await fetch(`/api/sessions/${sessionId}`, {
+        method: "PATCH",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ description: value }),
+      });
+      if (res.ok) {
+        lastSaved.current = value;
+        setStatus("saved");
+        router.refresh();
+      } else {
+        setStatus("idle");
+        toast.error("Could not save description");
+      }
+    }, AUTOSAVE_DELAY_MS);
+
+    return () => clearTimeout(timer);
+  }, [value, sessionId, router]);
+
+  useEffect(() => {
+    if (status !== "saved") return;
+    const timer = setTimeout(() => setStatus("idle"), 1500);
+    return () => clearTimeout(timer);
+  }, [status]);
+
+  return (
+    <div className="mt-1 flex items-start gap-2">
+      <textarea
+        value={value}
+        onChange={(e) => setValue(e.target.value)}
+        placeholder="Add a description..."
+        aria-label="Session description"
+        rows={1}
+        className="min-w-0 max-w-full flex-1 resize-none border-b-2 border-transparent bg-transparent text-sm text-muted-foreground outline-none transition-colors placeholder:text-muted-foreground/60 hover:border-b-border focus:border-b-primary focus:text-foreground [field-sizing:content]"
+      />
+      <div className="relative mt-0.5 flex size-4 shrink-0 items-center justify-center">
+        <Loader2
+          className={cn(
+            "absolute size-4 animate-spin text-muted-foreground transition-opacity",
+            status === "saving" ? "opacity-100" : "opacity-0",
+          )}
+        />
+        <Check
+          className={cn(
+            "absolute size-4 text-primary transition-opacity",
+            status === "saved" ? "opacity-100" : "opacity-0",
+          )}
+        />
+      </div>
+    </div>
+  );
+}