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/web/web.go

@@ -10,8 +10,8 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/scottyah/tracker/internal/config"
-	"github.com/scottyah/tracker/internal/store"
+	"github.com/scottyah/fit/internal/config"
+	"github.com/scottyah/fit/internal/store"
 )
 
 //go:embed templates/*.html