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/handlers.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/scottyah/tracker/internal/store"
+	"github.com/scottyah/fit/internal/store"
 )
 
 // gratitudeMax caps one gratitude line. It is a line, not an essay, and a cap
@@ -250,7 +250,7 @@ func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
 		s.fail(w, http.StatusInternalServerError, "The export did not build: "+err.Error())
 		return
 	}
-	name := "tracker-" + s.today() + ".json"
+	name := "fit-" + s.today() + ".json"
 	w.Header().Set("Content-Type", "application/json")
 	w.Header().Set("Content-Disposition", `attachment; filename="`+name+`"`)
 	w.Write(body)