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_users.go

@@ -53,7 +53,7 @@ func (d *DB) ListUsers() ([]User, error) {
 	}
 	defer rows.Close()
 
-	var users []User
+	users := make([]User, 0)
 	for rows.Next() {
 		user, err := scanUser(rows)
 		if err != nil {