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>

web/static/css/themes.css

@@ -1,5 +1,5 @@
 /*
- * TopDeck theme presets.
+ * Canon theme presets.
  *
  * Every color on the site comes from these variables, so a custom theme only
  * needs to override them (Settings → Appearance → Custom CSS), e.g.