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>

TODO.md

@@ -10,8 +10,8 @@
 
 ## Cosmetic
 
-- [ ] **Refactor the CSS.** Make it easy to re-theme by keeping the definitions of primary/background/highlight/etc colors confined to a few specific styles. If this requires adopting some kind of CSS framework/preprocessing, so be it.
-- [ ] **Re-styling**. The entire app could use a face lift, but in particular the splash page is ugly.
+- [x] **Refactor the CSS.** All colors/fonts/radii/shadows now live as tokens in `frontend/src/assets/css/theme.css`; everything else derives translucent variants via `color-mix()`. Re-theming the app = editing that one file (plus the mirrored token block at the top of `src/pirats/rules.html`).
+- [x] **Re-styling**. New "Lantern & Brine" theme: brass/parchment on lantern-lit ink-navy, Pirata One wordmark, Alegreya SC/Sans (self-hosted via fontsource), parchment-faced playing cards, rebuilt splash page.
 
 ## Worth Double-Checking / Minor