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/admin.go

@@ -5,7 +5,7 @@ import (
 	"html/template"
 	"net/http"
 
-	"github.com/cdnutter/topdeck/internal/store"
+	"github.com/cdnutter/canon/internal/store"
 )
 
 // adminPage carries a JSON blob the page script boots from.