fix: map manager crashes on empty maps directory (#280)

* fix: return empty array from ScanMaps when maps dir is missing

When the maps directory doesn't exist, ScanMaps returned nil which
Go serializes as JSON `null`. The frontend crashed trying to spread
null into an array, and the silent catch block redirected to `/`
with no console output — making the map manager appear broken.

Return `[]MapInfo{}` (JSON `[]`) instead, and log errors before
redirecting.

* fix: ensure ScanMaps never returns nil slice

Initialize maps as []MapInfo{} so an existing-but-empty directory
also serializes as JSON [] instead of null. Update test to match.

* fix: update handler test to expect [] instead of null

Author: fank

internal/maptool/scanner.go

@@ -49,7 +49,7 @@ func ScanMaps(mapsDir string) ([]MapInfo, error) {
 	entries, err := os.ReadDir(mapsDir)
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, nil
+			return []MapInfo{}, nil
 		}
 		return nil, fmt.Errorf("read maps dir: %w", err)
 	}
@@ -72,7 +72,7 @@ func ScanMaps(mapsDir string) ([]MapInfo, error) {
 		{"color-relief.pmtiles", "tiles"},
 	}
 
-	var maps []MapInfo
+	maps := []MapInfo{}
 	for _, entry := range entries {
 		if !entry.IsDir() {
 			continue

internal/maptool/scanner_test.go

@@ -87,7 +87,7 @@ func TestScanMaps_EmptyDir(t *testing.T) {
 func TestScanMaps_NonExistent(t *testing.T) {
 	maps, err := ScanMaps("/tmp/nonexistent-dir-12345")
 	require.NoError(t, err)
-	assert.Nil(t, maps)
+	assert.Empty(t, maps)
 }
 
 func TestScanMaps_NotADirectory(t *testing.T) {

internal/server/handler_maptool_test.go

@@ -86,8 +86,8 @@ func TestGetMapToolMaps_Empty(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, http.StatusOK, rec.Code)
 
-	// Empty maps dir returns null JSON (nil slice)
-	assert.Equal(t, "null", jsonTrimmed(rec))
+	// Empty maps dir returns empty JSON array
+	assert.Equal(t, "[]", jsonTrimmed(rec))
 }
 
 func TestGetMapToolMaps_WithMaps(t *testing.T) {

ui/src/pages/map-manager/MapManager.tsx

@@ -94,7 +94,8 @@ export function MapManager(): JSX.Element {
       ]);
       setTools(t);
       setMaps(m);
-    } catch {
+    } catch (err) {
+      console.error("Map manager failed to load:", err);
       navigate("/", { replace: true });
       return;
     }