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/PlexInfo.tsx

@@ -145,7 +145,7 @@ export default function PlexInfo({
             <img
               src={detail.thumb}
               alt=""
-              className={`shrink-0 rounded-xl border border-border object-cover ${overlay ? "w-24 sm:w-28" : "w-32 sm:w-44"} aspect-[2/3]`}
+              className={`shrink-0 rounded-xl border border-border object-cover ${overlay ? "w-24 sm:w-28" : "w-32 sm:w-44"} aspect-2/3`}
             />
 
             <div className={`min-w-0 flex-1 ${overlay ? "text-white/90" : ""}`}>
@@ -412,14 +412,14 @@ export default function PlexInfo({
                     disabled={!onPlayItem}
                     className="group/col w-28 shrink-0 text-left"
                   >
-                    <div className="relative aspect-[2/3] overflow-hidden rounded-lg border border-border bg-surface">
+                    <div className="relative aspect-2/3 overflow-hidden rounded-lg border border-border bg-surface">
                       <img
                         src={m.thumb}
                         alt=""
                         className="h-full w-full object-cover transition group-hover/col:scale-105"
                       />
                       {m.status === "watched" && (
-                        <span className="absolute right-1 top-1 rounded bg-black/70 px-1 text-[9px] text-white">
+                        <span className="absolute right-1 top-1 rounded-sm bg-black/70 px-1 text-[9px] text-white">
                           ✓
                         </span>
                       )}