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/DownloadCenter.tsx

@@ -181,7 +181,7 @@ function SourceRef({ url }: { url: string }) {
         onClick={copy}
         aria-label={t("downloads.source.copy")}
         title={t("downloads.source.copy")}
-        className="shrink-0 p-0.5 rounded hover:bg-surface hover:text-fg transition"
+        className="shrink-0 p-0.5 rounded-sm hover:bg-surface hover:text-fg transition"
       >
         <Copy className="w-3 h-3" />
       </button>
@@ -227,7 +227,7 @@ function DownloadRow({
       <div className="min-w-0 flex-1">
         <div className="font-medium leading-snug line-clamp-1 flex items-center gap-1.5">
           {job.job_kind === "edit" && (
-            <span className="inline-flex items-center gap-1 shrink-0 px-1.5 py-0.5 rounded text-[10px] font-semibold uppercase tracking-wide bg-accent/15 text-accent">
+            <span className="inline-flex items-center gap-1 shrink-0 px-1.5 py-0.5 rounded-sm text-[10px] font-semibold uppercase tracking-wide bg-accent/15 text-accent">
               <Scissors className="w-3 h-3" /> {t("downloads.clipBadge")}
             </span>
           )}
@@ -435,7 +435,7 @@ function ErrorModal({
               {t("downloads.errors.fail.detailsToggle")}
             </button>
             {showRaw && (
-              <pre className="mt-1.5 text-xs bg-surface rounded-lg p-2.5 overflow-x-auto whitespace-pre-wrap break-words max-h-40 text-muted">
+              <pre className="mt-1.5 text-xs bg-surface rounded-lg p-2.5 overflow-x-auto whitespace-pre-wrap wrap-break-word max-h-40 text-muted">
                 {job.error}
               </pre>
             )}
@@ -713,7 +713,7 @@ function AdminSystem() {
                 <span className="text-muted">{formatBytes(u.footprint_bytes)}</span>
                 <button
                   onClick={() => setQuotaFor({ id: u.user_id, email: u.email ?? "" })}
-                  className="p-1 rounded hover:bg-surface text-muted hover:text-fg"
+                  className="p-1 rounded-sm hover:bg-surface text-muted hover:text-fg"
                   title={t("downloads.admin.editQuota")}
                 >
                   <Settings2 className="w-4 h-4" />