build(deps): migrate Tailwind CSS v3.4 → v4

Root-fixes the opacity-modifier seed bug behind R10 S0: the theme colors were
bare `var(--x)` hex with no <alpha-value>, so `bg-accent/40`, `ring-accent/40`,
`border-border/60`, `text-muted/70`, … (~99 sites across 45 files) emitted no
CSS rule at all — latent no-ops. v4 implements the opacity modifier via
`color-mix`, so the modifier now works with the existing hex vars, no per-scheme
token duplication needed.

- `npx @tailwindcss/upgrade`: `@tailwind` directives → `@import "tailwindcss"`,
  JS config → CSS `@theme` (colors, --font-sans, the z-index tokens preserved),
  classname renames (rounded/shadow/blur scale, outline-none→outline-hidden,
  ring→ring-2) across 56 files; PostCSS plugin → @tailwindcss/postcss,
  autoprefixer removed (v4 prefixes for you).
- `@theme` → `@theme inline` (load-bearing): the color vars are redefined at
  runtime under html[data-scheme]/[data-theme]; inline resolves them at the use
  site so per-scheme opacity tints track the active scheme (the shadcn v4 pattern).
- Revert one codemod false-positive: it rewrote the English word "rounded" →
  "rounded-sm" inside a releaseNotes.ts prose sentence.

Gate green: tsc(app+node) / eslint 0-err / prettier / vitest 79 / knip.
Verified in-browser: bg-accent/15 now computes a scheme-tracking color at 0.15
alpha across all 6 schemes × 2 themes; the hand-rolled glass border/shadow/
radius/backdrop-filter render unchanged under v4.

frontend/src/components/AddToPlaylist.tsx

@@ -124,7 +124,7 @@ export default function AddToPlaylist({
                 // the floating element only). `toggle()` already no-ops while busy, so re-entrancy is
                 // guarded without touching focus; `aria-busy` on the list conveys the pending state.
                 label={playlistName(pl, t)}
-                className="w-full flex items-center gap-2 text-sm px-1.5 py-1.5 rounded-lg text-fg outline-none transition hover:bg-card/60 data-[active]:bg-[color-mix(in_srgb,var(--accent)_24%,transparent)]"
+                className="w-full flex items-center gap-2 text-sm px-1.5 py-1.5 rounded-lg text-fg outline-hidden transition hover:bg-card/60 data-active:bg-[color-mix(in_srgb,var(--accent)_24%,transparent)]"
               >
                 <span
                   className={`grid place-items-center w-4 h-4 rounded border ${
@@ -156,7 +156,7 @@ export default function AddToPlaylist({
                 }
               }}
               placeholder={t("playlists.newPlaylist")}
-              className="flex-1 min-w-0 bg-card border border-border rounded-md px-2 py-1 text-xs outline-none focus:border-accent"
+              className="flex-1 min-w-0 bg-card border border-border rounded-md px-2 py-1 text-xs outline-hidden focus:border-accent"
             />
           </div>
         </div>