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/package.json

@@ -37,12 +37,12 @@
   "devDependencies": {
     "@eslint/js": "9.39.1",
     "@playwright/test": "^1.61.1",
+    "@tailwindcss/postcss": "^4.3.3",
     "@testing-library/react": "^16.3.2",
     "@types/node": "^26.1.1",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
-    "autoprefixer": "^10.4.19",
     "eslint": "9.39.1",
     "eslint-config-prettier": "10.1.8",
     "eslint-plugin-react-hooks": "5.2.0",
@@ -52,7 +52,7 @@
     "knip": "6.27.0",
     "postcss": "^8.4.39",
     "prettier": "3.6.2",
-    "tailwindcss": "^3.4.6",
+    "tailwindcss": "^4.3.3",
     "typescript": "^5.5.3",
     "typescript-eslint": "8.46.0",
     "vite": "^5.3.4",