Add multi-user support with export feature

- New users table (migration 004) with user_id on exercises, training_sets, sessions
- User CRUD endpoints (GET/POST /api/v1/users, DELETE /api/v1/users/{id})
- All existing endpoints scoped to X-User-ID header
- CSV export endpoint (GET /api/v1/export) for completed sessions
- UserGate in PageShell: blocks app until a user is selected
- Settings page for managing users (create, switch, delete)
- BottomNav/Sidebar updated with settings navigation
- Fix: nil pointer panic in handleDeleteUser on success path
- Fix: export download now uses fetch with X-User-ID header instead of window.location.href

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

backend/internal/handler/stats.go

@@ -3,13 +3,18 @@ package handler
 import "net/http"
 
 func (h *Handler) handleGetLastLog(w http.ResponseWriter, r *http.Request) {
+	uid, err := userID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
 	id, err := pathID(r, "id")
 	if err != nil {
 		writeError(w, http.StatusBadRequest, "Ungültige ID")
 		return
 	}
 
-	lastLog, err := h.store.GetLastLog(id)
+	lastLog, err := h.store.GetLastLog(id, uid)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, "Fehler beim Laden des letzten Logs")
 		return
@@ -22,6 +27,11 @@ func (h *Handler) handleGetLastLog(w http.ResponseWriter, r *http.Request) {
 }
 
 func (h *Handler) handleGetExerciseHistory(w http.ResponseWriter, r *http.Request) {
+	uid, err := userID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
 	id, err := pathID(r, "id")
 	if err != nil {
 		writeError(w, http.StatusBadRequest, "Ungültige ID")
@@ -29,7 +39,7 @@ func (h *Handler) handleGetExerciseHistory(w http.ResponseWriter, r *http.Reques
 	}
 	limit := queryInt(r, "limit", 30)
 
-	logs, err := h.store.GetExerciseHistory(id, limit)
+	logs, err := h.store.GetExerciseHistory(id, uid, limit)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, "Fehler beim Laden der Übungshistorie")
 		return
@@ -38,7 +48,13 @@ func (h *Handler) handleGetExerciseHistory(w http.ResponseWriter, r *http.Reques
 }
 
 func (h *Handler) handleGetStatsOverview(w http.ResponseWriter, r *http.Request) {
-	overview, err := h.store.GetStatsOverview()
+	uid, err := userID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	overview, err := h.store.GetStatsOverview(uid)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, "Fehler beim Laden der Statistiken")
 		return