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/common.js

@@ -1,4 +1,4 @@
-// Shared helpers for TopDeck's pages. No build step, no dependencies.
+// Shared helpers for Canon's pages. No build step, no dependencies.
 
 /** Create an element: h("button", { class: "btn", onclick }, "Save") */
 export function h(tag, attrs = {}, ...children) {
@@ -53,7 +53,7 @@ export class APIError extends Error {
 
 /** Call the admin JSON API. Throws APIError with a human message. */
 export async function api(method, path, body) {
-  const opts = { method, headers: { "X-TopDeck": "1" }, credentials: "same-origin" };
+  const opts = { method, headers: { "X-Canon": "1" }, credentials: "same-origin" };
   if (body instanceof FormData) {
     opts.body = body;
   } else if (body !== undefined) {