Apply the theme: tokens, Oswald, song cards, toasts

The theme handoff encoded as CSS custom properties rather than a Tailwind
config, since there is no Tailwind here. Palette, spacing, radii, shadows and
motion follow it as written; docs/theme.md lists what differs and why.

- Oswald vendored as a 21KB variable woff2, latin subset, no CDN. Its phantom
  weight 900 resolved to 700 — loading a weight you don't have is what made
  the brand render differently per platform
- color-scheme: dark makes the native audio element fit the palette, which
  was the handoff's complaint about it
- Songs are text cards rather than artwork tiles, because there is no
  artwork. The unreviewed state keeps its red-brown border and gains a badge,
  so it is never carried by colour alone
- Nav is the three-column grid; the mobile menu is <details>, no JS
- Flash messages became bottom-right toasts

Favicon carried over from the Nuxt project.

README.md

@@ -16,6 +16,7 @@ Invite-only, no public registration. Built for about ten friends.
 | [CONTEXT.md](CONTEXT.md) | The glossary — every domain term, in English and Finnish |
 | [docs/spec.md](docs/spec.md) | What the app does: rules, pipeline, routes, API contract, schema |
 | [docs/decisions.md](docs/decisions.md) | Why it is that way. Append-only |
+| [docs/theme.md](docs/theme.md) | The visual language: tokens, type, and what differs from the theme handoff |
 | [docs/later.md](docs/later.md) | Deliberately not in v1, with the reasoning kept |
 
 ## Branches