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/export.go

@@ -0,0 +1,42 @@
+package handler
+
+import (
+	"encoding/csv"
+	"net/http"
+	"strconv"
+	"time"
+)
+
+// handleExport gibt alle Trainingsdaten eines Nutzers als CSV zurück.
+func (h *Handler) handleExport(w http.ResponseWriter, r *http.Request) {
+	uid, err := userID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	rows, err := h.store.ExportLogs(uid)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "Export fehlgeschlagen")
+		return
+	}
+
+	filename := "training-export-" + time.Now().Format("2006-01-02") + ".csv"
+	w.Header().Set("Content-Type", "text/csv; charset=utf-8")
+	w.Header().Set("Content-Disposition", "attachment; filename=\""+filename+"\"")
+
+	cw := csv.NewWriter(w)
+	cw.Write([]string{"datum", "uebung", "satz", "gewicht_kg", "wiederholungen", "notiz"})
+
+	for _, row := range rows {
+		cw.Write([]string{
+			row.SessionStarted.Format("2006-01-02 15:04"),
+			row.ExerciseName,
+			strconv.Itoa(row.SetNumber),
+			strconv.FormatFloat(row.WeightKg, 'f', 2, 64),
+			strconv.Itoa(row.Reps),
+			row.Note,
+		})
+	}
+	cw.Flush()
+}