Add HTML rulebook page with always-available Rules link

New /#/rules route renders the full rulebook transcribed from
"Rats with Gats beta.pdf" (intro, character creation, gameplay,
obstacle charts, challenge resolution, example of play, and setting
info), with an in-page table of contents. A fixed corner link in
App.svelte opens it in a new tab from every page, and the splash
page's previously broken /rules link now points at the new route.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/Home.svelte

@@ -56,6 +56,6 @@
     </div>
 
     <div class="links mt-4">
-        <a href="/rules" class="text-muted">Read the Rules</a>
+        <a href="#/rules" class="text-muted">Read the Rules</a>
     </div>
 </div>