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/LanguageSwitcher.tsx

@@ -56,7 +56,7 @@ export default function LanguageSwitcher({
         {rail ? (
           /* Current-language badge (HU/EN) so the active language reads at a glance without
              opening the menu. The ring matches the page background to detach it from the icon. */
-          <span className="absolute -bottom-1 -right-1 text-[8px] font-bold leading-none px-1 py-[1px] rounded bg-accent text-accent-fg ring-2 ring-bg">
+          <span className="absolute -bottom-1 -right-1 text-[8px] font-bold leading-none px-1 py-px rounded-sm bg-accent text-accent-fg ring-2 ring-bg">
             {current.code.toUpperCase()}
           </span>
         ) : (
@@ -78,7 +78,7 @@ export default function LanguageSwitcher({
               setOpen(false);
             }}
             label={l.label}
-            className="w-full flex items-center justify-between gap-2 text-sm px-2 py-2 rounded-lg text-muted outline-none transition hover:text-fg hover:bg-card data-[active]:bg-[color-mix(in_srgb,var(--accent)_24%,transparent)] data-[active]:text-fg"
+            className="w-full flex items-center justify-between gap-2 text-sm px-2 py-2 rounded-lg text-muted outline-hidden transition hover:text-fg hover:bg-card data-active:bg-[color-mix(in_srgb,var(--accent)_24%,transparent)] data-active:text-fg"
           >
             <span>{l.label}</span>
             {l.code === value && <Check className="w-4 h-4 text-accent" />}