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>