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>

cmd/server/main.go

@@ -14,9 +14,9 @@ import (
 	// correctly inside a scratch image, which has no /usr/share/zoneinfo.
 	_ "time/tzdata"
 
-	"github.com/scottyah/tracker/internal/config"
-	"github.com/scottyah/tracker/internal/store"
-	"github.com/scottyah/tracker/internal/web"
+	"github.com/scottyah/fit/internal/config"
+	"github.com/scottyah/fit/internal/store"
+	"github.com/scottyah/fit/internal/web"
 )
 
 func main() {