Add exercise numbers, image uploads, version display, session resume, and training sparklines

- Exercise number (UF#): optional field on exercises, displayed in cards, training, and sets
- Import training plan numbers via migration 005 (UPDATE by name)
- Exercise images: JPG upload with multi-image support per exercise (migration 006)
- Version endpoint (GET /api/v1/version) with ldflags injection in Makefile and Dockerfile
- Version displayed on settings page
- Session resume: GET /api/v1/sessions/active endpoint, auto-resume on training page load
- Block new session while one is active (409 Conflict)
- e1RM sparkline chart per exercise during training (Epley formula)
- Fix CORS: add X-User-ID to allowed headers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/session.go

@@ -7,6 +7,36 @@ import (
 	"strings"
 )
 
+func (h *Handler) handleGetActiveSession(w http.ResponseWriter, r *http.Request) {
+	uid, err := userID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	session, err := h.store.GetActiveSession(uid)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "Fehler beim Suchen der aktiven Session")
+		return
+	}
+	if session == nil {
+		writeJSON(w, http.StatusOK, nil)
+		return
+	}
+
+	// Exercises des Sets mitliefern
+	exercises, err := h.store.GetSetExercises(session.SetID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "Fehler beim Laden der Übungen")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"session":   session,
+		"exercises": exercises,
+	})
+}
+
 func (h *Handler) handleCreateSession(w http.ResponseWriter, r *http.Request) {
 	uid, err := userID(r)
 	if err != nil {
@@ -26,6 +56,10 @@ func (h *Handler) handleCreateSession(w http.ResponseWriter, r *http.Request) {
 
 	session, err := h.store.CreateSession(uid, req.SetID)
 	if err != nil {
+		if strings.Contains(err.Error(), "SESSION_OPEN") {
+			writeError(w, http.StatusConflict, "Es läuft bereits ein Training. Bitte zuerst beenden.")
+			return
+		}
 		if strings.Contains(err.Error(), "existiert nicht") {
 			writeError(w, http.StatusBadRequest, err.Error())
 			return