Rename the app from tracker to fit

The directory, the repo and the hostname are all fit; the module path,
the image, the namespace and the data file were still tracker. Nothing
is deployed yet, so this is free now and would not be later.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/store.go

@@ -284,7 +284,7 @@ func (s *Store) Export() ([]byte, error) {
 func (s *Store) Import(body []byte) error {
 	var next data
 	if err := json.Unmarshal(body, &next); err != nil {
-		return fmt.Errorf("that file is not a tracker export: %w", err)
+		return fmt.Errorf("that file is not a fit export: %w", err)
 	}
 	if next.Days == nil {
 		next.Days = map[string]*Day{}
@@ -313,7 +313,7 @@ func (s *Store) persist() error {
 	}
 
 	dir := filepath.Dir(s.path)
-	tmp, err := os.CreateTemp(dir, ".tracker-*.json")
+	tmp, err := os.CreateTemp(dir, ".fit-*.json")
 	if err != nil {
 		return fmt.Errorf("create temp file: %w", err)
 	}