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/js/public.js

@@ -75,7 +75,7 @@ document.addEventListener("click", (e) => {
 // Grid / list toggle on list pages, remembered per list in this browser.
 const toggle = document.querySelector("[data-layout-toggle]");
 if (toggle) {
-  const key = "topdeck:layout:" + toggle.dataset.key;
+  const key = "canon:layout:" + toggle.dataset.key;
   const apply = (layout) => {
     for (const { ol } of lists) {
       ol.classList.toggle("items--grid", layout === "grid");