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/ChannelDiscovery.tsx

@@ -205,7 +205,7 @@ export default function ChannelDiscovery({
     <>
       {/* Intro + the table controls are fixed chrome; only the table rows scroll (sticky header). */}
       <PageToolbar>
-        <div className="px-4 pt-3 max-w-[96rem] mx-auto flex items-start justify-between gap-4">
+        <div className="px-4 pt-3 max-w-384 mx-auto flex items-start justify-between gap-4">
           <p className="text-xs text-muted leading-relaxed">{t("channels.discovery.intro")}</p>
           {/* Cards has no column headers, so a dropdown drives the sort + a pager (table uses its own). */}
           {layout === "cards" && (
@@ -224,7 +224,7 @@ export default function ChannelDiscovery({
           )}
         </div>
       </PageToolbar>
-      <div className="px-4 pb-4 max-w-[96rem] mx-auto">
+      <div className="px-4 pb-4 max-w-384 mx-auto">
         {query.isError && !query.data ? (
           <StateMessage tone="error" onRetry={() => query.refetch()}>
             {t("common.loadError")}
@@ -239,7 +239,7 @@ export default function ChannelDiscovery({
             persistKey={accountKey(LS.channelDiscoveryTable) ?? undefined}
             controlsPosition="both"
             controlsInBand
-            controlsBandClassName="px-4 max-w-[96rem] mx-auto"
+            controlsBandClassName="px-4 max-w-384 mx-auto"
             sort={sort}
             onSortChange={setSort}
             emptyText={t("channels.discovery.empty")}