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/ChannelPage.tsx

@@ -297,7 +297,7 @@ export default function ChannelPage({
           <button
             data-testid="channel-back"
             onClick={closeChannel}
-            className="absolute top-5 left-7 sm:left-9 z-base inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm bg-black/45 text-white hover:bg-black/65 backdrop-blur-sm"
+            className="absolute top-5 left-7 sm:left-9 z-base inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm bg-black/45 text-white hover:bg-black/65 backdrop-blur-xs"
           >
             <ArrowLeft className="w-4 h-4" />
             {t("channel.back")}