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_test.go

@@ -9,8 +9,8 @@ import (
 	"testing"
 	"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"
 )
 
 // newServer builds a server whose "today" is fixed, so the day arithmetic is
@@ -22,7 +22,7 @@ func newServer(t *testing.T, today string) (*Server, *store.Store) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	st, err := store.Open(filepath.Join(t.TempDir(), "tracker.json"))
+	st, err := store.Open(filepath.Join(t.TempDir(), "fit.json"))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -240,11 +240,11 @@ func TestExportThenImport(t *testing.T) {
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status %d", rec.Code)
 	}
-	if !strings.Contains(rec.Header().Get("Content-Disposition"), "tracker-2026-07-31.json") {
+	if !strings.Contains(rec.Header().Get("Content-Disposition"), "fit-2026-07-31.json") {
 		t.Errorf("download name = %q", rec.Header().Get("Content-Disposition"))
 	}
 
-	fresh, err := store.Open(filepath.Join(t.TempDir(), "tracker.json"))
+	fresh, err := store.Open(filepath.Join(t.TempDir(), "fit.json"))
 	if err != nil {
 		t.Fatal(err)
 	}