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/config/config.go

@@ -24,7 +24,7 @@ func Load() (*Config, error) {
 	c := &Config{
 		Addr:     env("ADDR", ":8080"),
 		BaseURL:  strings.TrimRight(env("BASE_URL", "http://localhost:8080"), "/"),
-		DataPath: env("DATA_PATH", "data/tracker.json"),
+		DataPath: env("DATA_PATH", "data/fit.json"),
 	}
 
 	// The whole app is date arithmetic, so a wrong timezone silently logs