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/PlexCollectionEditor.tsx

@@ -131,7 +131,7 @@ export default function PlexCollectionEditor({
           onChange={(e) => setNewName(e.target.value)}
           onKeyDown={(e) => e.key === "Enter" && createNew()}
           placeholder={t("plex.collEditor.newPlaceholder")}
-          className="glass-card min-w-0 flex-1 rounded-lg px-3 py-2 text-sm outline-none focus:border-accent"
+          className="glass-card min-w-0 flex-1 rounded-lg px-3 py-2 text-sm outline-hidden focus:border-accent"
         />
         <button
           onClick={createNew}
@@ -149,7 +149,7 @@ export default function PlexCollectionEditor({
           value={q}
           onChange={(e) => setQ(e.target.value)}
           placeholder={t("plex.collEditor.search")}
-          className="glass-card mb-2 w-full rounded-lg px-3 py-1.5 text-sm outline-none focus:border-accent"
+          className="glass-card mb-2 w-full rounded-lg px-3 py-1.5 text-sm outline-hidden focus:border-accent"
         />
       )}
 
@@ -193,7 +193,7 @@ export default function PlexCollectionEditor({
                 <button
                   onClick={() => removeCollection(c.id, c.title)}
                   disabled={!!busy}
-                  className="shrink-0 rounded px-1.5 py-0.5 text-xs text-muted hover:text-red-400 disabled:opacity-50"
+                  className="shrink-0 rounded-sm px-1.5 py-0.5 text-xs text-muted hover:text-red-400 disabled:opacity-50"
                 >
                   {t("plex.collEditor.delete")}
                 </button>
@@ -226,7 +226,7 @@ export default function PlexCollectionEditor({
                 <button
                   onClick={() => markEditable(c.id)}
                   disabled={!!busy}
-                  className="glass-card glass-hover shrink-0 rounded px-2 py-0.5 text-xs disabled:opacity-50"
+                  className="glass-card glass-hover shrink-0 rounded-sm px-2 py-0.5 text-xs disabled:opacity-50"
                 >
                   {t("plex.collEditor.makeEditable")}
                 </button>