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.go

@@ -114,7 +114,7 @@ func (d *DB) ListDirty() ([]DirtyModule, error) {
 	}
 	defer rows.Close()
 
-	var modules []DirtyModule
+	modules := make([]DirtyModule, 0)
 	for rows.Next() {
 		var module DirtyModule
 		var markedAt string
@@ -158,7 +158,7 @@ func (d *DB) ListApplyLog(limit int) ([]ApplyLogEntry, error) {
 	}
 	defer rows.Close()
 
-	var entries []ApplyLogEntry
+	entries := make([]ApplyLogEntry, 0)
 	for rows.Next() {
 		var entry ApplyLogEntry
 		var success int
@@ -182,7 +182,7 @@ func (d *DB) ListSambaShares() ([]SambaShare, error) {
 	}
 	defer rows.Close()
 
-	var shares []SambaShare
+	shares := make([]SambaShare, 0)
 	for rows.Next() {
 		share, err := scanSambaShare(rows)
 		if err != nil {