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.

docs/later.md

@@ -130,6 +130,27 @@ Also here: pruning the count-based leaderboards once the queue has drained and t
 
 ---
 
+## Review form as a mixer channel
+
+Idea for the UI polish pass, not now: put the score slider and the review textarea **on one row**,
+with the slider **vertical** like a channel fader on a mixing desk. The score stops being a form
+field and becomes the instrument the app is actually about, and the two things you do at once —
+decide a number, write why — stop being stacked a screenful apart.
+
+Notes for whoever builds it:
+
+- A vertical `<input type="range">` is native now: `writing-mode: vertical-lr; direction: rtl` gives
+  bottom-to-top travel with no JS and no custom widget, so keyboard support and the value stay free.
+- Keep the live `<output>` — on a fader it wants to sit at the top of the track, reading like a
+  channel's gain display.
+- The row needs a mobile answer: below ~640px, either keep the fader and shrink the textarea beside
+  it, or fall back to the current stacked layout. A short vertical fader is worse than a horizontal
+  one, so measure before choosing.
+- Tick marks along the track (1 / 25 / 50 / 75 / 100) replace today's `.scorescale` row, and are
+  what make it read as equipment rather than decoration.
+
+---
+
 ## Filters on the browse list
 
 `/songs` is newest-first with no filters. Once there are a couple of hundred songs, "which ones