feat: restore Langfuse observability integration (#103)

- Add lib/langfuse.ts with client, trace input/output, telemetry config
- Add instrumentation.ts for OpenTelemetry setup with Langfuse span processor
- Add /api/log-save endpoint for logging diagram saves
- Add /api/log-feedback endpoint for thumbs up/down feedback
- Update chat route with sessionId tracking and telemetry
- Add feedback buttons (thumbs up/down) to chat messages
- Add sessionId tracking throughout the app
- Update env.example with Langfuse configuration
- Add @langfuse/client, @langfuse/otel, @langfuse/tracing, @opentelemetry/sdk-trace-node

components/chat-panel.tsx

@@ -63,6 +63,9 @@ export default function ChatPanel({
     const [showHistory, setShowHistory] = useState(false);
     const [input, setInput] = useState("");
 
+    // Generate a unique session ID for Langfuse tracing
+    const [sessionId, setSessionId] = useState(() => `session-${Date.now()}-${Math.random().toString(36).slice(2, 9)}`);
+
     // Store XML snapshots for each user message (keyed by message index)
     const xmlSnapshotsRef = useRef<Map<number, string>>(new Map());
 
@@ -217,6 +220,7 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
                     {
                         body: {
                             xml: chartXml,
+                            sessionId,
                         },
                     }
                 );
@@ -297,6 +301,7 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
             {
                 body: {
                     xml: savedXml,
+                    sessionId,
                 },
             }
         );
@@ -353,6 +358,7 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
             {
                 body: {
                     xml: savedXml,
+                    sessionId,
                 },
             }
         );
@@ -440,6 +446,7 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
                     error={error}
                     setInput={setInput}
                     setFiles={handleFileChange}
+                    sessionId={sessionId}
                     onRegenerate={handleRegenerate}
                     onEditMessage={handleEditMessage}
                 />
@@ -455,12 +462,14 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
                     onClearChat={() => {
                         setMessages([]);
                         clearDiagram();
+                        setSessionId(`session-${Date.now()}-${Math.random().toString(36).slice(2, 9)}`);
                         xmlSnapshotsRef.current.clear();
                     }}
                     files={files}
                     onFileChange={handleFileChange}
                     showHistory={showHistory}
                     onToggleHistory={setShowHistory}
+                    sessionId={sessionId}
                     error={error}
                 />
             </footer>