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/song.html

@@ -1,10 +1,12 @@
 {{define "content"}}
 {{$s := .Data}}
 <h1>{{$s.Title}}</h1>
-<p class="byline">
-  {{$s.Artist}} · {{$s.GenreLabel}} · {{$s.Length}} ·
-  lähettänyt {{$s.Submitter}} {{fidate $s.CreatedAt}}
-  {{if $s.Own}}<span class="tag">oma kappale</span>{{end}}
+<p class="songmeta">
+  <strong>{{$s.Artist}}</strong>
+  <span class="badge genre">{{$s.GenreLabel}}</span>
+  <span>{{$s.Length}}</span>
+  <span>lähettänyt {{$s.Submitter}} {{fidate $s.CreatedAt}}</span>
+  {{if $s.Own}}<span class="badge admin">oma kappale</span>{{end}}
 </p>
 
 {{template "player" $s}}
@@ -31,7 +33,7 @@
   </form>
   <form method="post" action="/songs/{{$s.ID}}/delete"
         onsubmit="return confirm('Poistetaanko kappale lopullisesti?')">
-    <button type="submit" class="danger">Poista kappale</button>
+    <button type="submit" class="danger ghost">Poista kappale</button>
   </form>
   <p class="muted small">Muokkaus ja poisto ovat mahdollisia vain ennen ensimmäistä arvostelua.</p>
 </details>
@@ -85,12 +87,12 @@
   {{if $s.Revealed}}
     {{if $s.Average}}<p class="average">Keskiarvo <strong>{{score $s.Average}}</strong></p>{{end}}
     {{range $s.Reviews}}
-      <article class="review">
+      <article class="review{{if .Own}} own{{end}}">
         <header>
-          <span class="avatar">{{.Initials}}</span>
-          <strong>{{.Reviewer}}</strong>
+          <span class="avatar small">{{.Initials}}</span>
+          <span class="who">{{.Reviewer}}</span>
           <span class="score">{{.Score}}</span>
-          <span class="muted small">{{fidate .CreatedAt}}</span>
+          <span class="meta">{{fidate .CreatedAt}}</span>
         </header>
         <p>{{.Text}}</p>
       </article>