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/render_test.go

@@ -0,0 +1,11 @@
+package server
+
+import "testing"
+
+func TestDefaultTitle(t *testing.T) {
+	for in, want := range map[string]string{"": "Canon", "  ": "Canon", "Chris": "Chris's Canon", " Ana ": "Ana's Canon"} {
+		if got := DefaultTitle(in); got != want {
+			t.Errorf("DefaultTitle(%q) = %q, want %q", in, got, want)
+		}
+	}
+}