Fork Notesium source and restructure into Go package layout

Initial fork of github.com/alonswartz/notesium into librenotes:
- Source moved to internal/notesium/ (package notesium)
- Thin entry point at cmd/librenotes/main.go
- Module renamed to git.librete.ch/public/librenotes
- main() exposed as notesium.Run()
- LICENSE preserved (MIT), NOTICE added with attribution
- Web assets and completion.bash co-located with embedding code
  to satisfy go:embed path constraints

Closes #3, #34, #35.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/notesium/heartbeat.go

@@ -0,0 +1,51 @@
+package notesium
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net/http"
+	"sync"
+	"time"
+)
+
+var (
+	mu            sync.Mutex
+	lastHeartbeat time.Time
+)
+
+func updateHeartbeat() {
+	mu.Lock()
+	lastHeartbeat = time.Now()
+	mu.Unlock()
+}
+
+func heartbeatH(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		updateHeartbeat()
+		next.ServeHTTP(w, r)
+	})
+}
+
+func heartbeatF(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		updateHeartbeat()
+		next(w, r)
+	}
+}
+
+func checkHeartbeat(server *http.Server) {
+	for {
+		time.Sleep(5 * time.Second)
+		mu.Lock()
+		if time.Since(lastHeartbeat) > 10*time.Second {
+			fmt.Println("No active client, stopping server.")
+			mu.Unlock()
+			if err := server.Shutdown(context.Background()); err != nil {
+				log.Fatalf("Server shutdown failed: %+v", err)
+			}
+			break
+		}
+		mu.Unlock()
+	}
+}