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.

templates/songs.html

@@ -2,12 +2,9 @@
 <h1>Kappaleet</h1>
 
 {{if .Data.Items}}
-  <table>
-    <thead><tr><th>Kappale</th><th>Genre</th><th>Kesto</th><th>Pisteet</th></tr></thead>
-    <tbody>
-      {{range .Data.Items}}{{template "songrow" .}}{{end}}
-    </tbody>
-  </table>
+  <div class="songgrid">
+    {{range .Data.Items}}{{template "songcard" .}}{{end}}
+  </div>
   {{with .Data.NextCursor}}<p><a href="/songs?cursor={{.}}">Vanhempia →</a></p>{{end}}
 {{else}}
   <p class="empty">Yhtään kappaletta ei ole vielä julkaistu.</p>