Re-theme the app: token-based CSS and a full face lift

CSS refactor: every color, font, radius, and shadow is now defined once
in theme.css; all other styles derive translucent variants with
color-mix(), so re-theming means editing one file. Removed the
duplicate core.css/style.css import chains, dead styles, and the
Bootstrap/Tailwind orphan classes that were silently unstyled
(the main reason the splash page looked broken).

New "Lantern & Brine" look: brass and parchment on a lantern-lit
ink-navy night, Pirata One wordmark with Alegreya SC/Sans body fonts
(self-hosted via fontsource — they were previously referenced but
never loaded), parchment-faced playing cards, and a rebuilt splash
page. Inline color styles in components were replaced with semantic
classes (prompt-box, notice-banner, status chips, etc.), the rulebook's
embedded palette was updated to match, and the leftover Vite favicon
and "frontend" page title were replaced.

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

frontend/package.json

@@ -14,6 +14,9 @@
     "vite": "^8.0.12"
   },
   "dependencies": {
+    "@fontsource/alegreya-sans": "^5.2.8",
+    "@fontsource/alegreya-sc": "^5.2.8",
+    "@fontsource/pirata-one": "^5.2.8",
     "svelte-spa-router": "^5.1.0"
   }
 }