Rename project from TopDeck to Canon

- Module github.com/cdnutter/canon, binary/command cmd/canon, Docker
  image and compose project "canon"
- Environment variables now use the CANON_ prefix
- Default site title is "<Your name>'s Canon" (or "Canon" with no name);
  a Site title set in Settings still overrides it
- Existing topdeck.db (and WAL files) is renamed to canon.db on start
- README: upgrade notes, including moving the old compose volume

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/authhandlers.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/cdnutter/topdeck/internal/auth"
+	"github.com/cdnutter/canon/internal/auth"
 )
 
 type loginPage struct {
@@ -143,7 +143,7 @@ func (s *Server) handleSetup(w http.ResponseWriter, r *http.Request) {
 	}
 	if s.setupCode == "" || subtle.ConstantTimeCompare([]byte(code), []byte(normalizeCode(s.setupCode))) != 1 {
 		s.loginLimit.Fail(ip)
-		fail(http.StatusForbidden, "That setup code isn't right. It's printed in the server log (docker compose logs topdeck).")
+		fail(http.StatusForbidden, "That setup code isn't right. It's printed in the server log (docker compose logs canon).")
 		return
 	}
 	if page.Username == "" || len(page.Username) > 64 {