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/app.css

@@ -1,5 +1,5 @@
 /* ==========================================================================
-   TopDeck. Colors and fonts come from themes.css.
+   Canon. Colors and fonts come from themes.css.
    ========================================================================== */
 
 *, *::before, *::after { box-sizing: border-box; }