fix: nil slice -> empty slice in list API responses

Backend:
- ListStorageJobs, ListSambaShares, ListNFSExports, ListUsers,
  ListDirty, ListApplyLog, ListWatchedMounts: initialize with
  make([]T, 0) instead of var x []T to avoid JSON null on empty.
  Fixes "Cannot read properties of null" crash on /storage.

Frontend:
- Storage.tsx: defensive setJobs(res.jobs ?? []) to guard against
  API returning null.

Tests:
- Add TestListStorageJobsEmptyReturnsSlice.

Version: 0.7.1

internal/db/queries_storage_test.go

@@ -0,0 +1,27 @@
+package db
+
+import (
+	"testing"
+)
+
+func TestListStorageJobsEmptyReturnsSlice(t *testing.T) {
+	d, err := Open(":memory:")
+	if err != nil {
+		t.Fatalf("open db: %v", err)
+	}
+	defer d.Close()
+	if err := d.Migrate(); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+
+	jobs, err := d.ListStorageJobs(50)
+	if err != nil {
+		t.Fatalf("ListStorageJobs: %v", err)
+	}
+	if jobs == nil {
+		t.Error("ListStorageJobs returned nil, want empty slice")
+	}
+	if len(jobs) != 0 {
+		t.Errorf("len(jobs) = %d, want 0", len(jobs))
+	}
+}