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/submission.html

@@ -1,5 +1,5 @@
 {{define "submission-status"}}
-<div class="status" {{if not .Done}}hx-get="/submit/{{.ID}}/status" hx-trigger="every 2s" hx-swap="outerHTML"{{end}}>
+<div class="status{{if .Failed}} failed{{end}}" {{if not .Done}}hx-get="/submit/{{.ID}}/status" hx-trigger="every 2s" hx-swap="outerHTML"{{end}}>
   <p class="{{if .Failed}}error{{end}}"><strong>{{.Label}}</strong></p>
   {{if .Failed}}
     {{with .StatusMsg}}<p class="muted small">{{.}}</p>{{end}}