Code-Cleanup: toter Check, none-Sentinel, Login-Jitter raus

- entry.go: redundantes stored != "" entfernt
- filepath-Sentinel "none" -> leerer String (Go-Zero-Value),
  Checks in entry.go/user.go und web/js/app.js vereinfacht
- auth.go: zufaelligen Login-Jitter samt loginJitter-Variable und
  Test-Seam entfernt (war nur Spielerei, kein echter Schutz)
- notes/migrations.md: einmalige "none"->"" Migration dokumentiert

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

entry.go

@@ -75,16 +75,16 @@ func handleCreateEntry(w http.ResponseWriter, r *http.Request) {
 		content = content[:1000]
 	}
 
-	filepath := "none"
+	filepath := ""
 	if r.MultipartForm != nil {
 		if files := r.MultipartForm.File["file"]; len(files) > 0 {
-			if stored, err := storeImage(files[0]); err == nil && stored != "" {
+			if stored, err := storeImage(files[0]); err == nil {
 				filepath = stored
 			}
 		}
 	}
 
-	if filepath == "none" && content == "" {
+	if filepath == "" && content == "" {
 		writeError(w, http.StatusBadRequest, "Leerer Beitrag!")
 		return
 	}