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/templates/pages/setup.html

@@ -2,14 +2,14 @@
 {{define "content"}}
 <section class="auth-card">
   <div class="auth-mark">{{icon "deck"}}</div>
-  <h1>Welcome to TopDeck</h1>
+  <h1>Welcome to Canon</h1>
   <p class="muted">Create the owner account. Only you can sign in; everyone else just sees your lists.</p>
   {{with .Data.Error}}<p class="alert alert--error" role="alert">{{icon "alert"}}{{.}}</p>{{end}}
   <form method="post" action="/setup" class="stack">
     <label class="field">
       <span class="field-label">Setup code</span>
       <input name="code" required autocomplete="one-time-code" placeholder="XXXX-XXXX" spellcheck="false" autocapitalize="characters">
-      <span class="field-hint">Printed in the server log when TopDeck starts. Run <code>docker compose logs topdeck</code> to see it.</span>
+      <span class="field-hint">Printed in the server log when Canon starts. Run <code>docker compose logs canon</code> to see it.</span>
     </label>
     <label class="field">
       <span class="field-label">Username</span>