fix(web): UX/a11y polish + middleware metadata route fix

- footer.tsx: Discord link → canonical discord.gg/PUwSMR9XNk
  (was outdated invite that 404'd).
- [locale]/layout.tsx: hooks count 40 → 54 (matches AGENTS.md).
- lib/stats.ts:88: drop '!' non-null assertion; use '??' fallback.
  Removes the only '!' in packages/web (project rule).
- nav-header.tsx: hamburger Button size='icon' → explicit h-11 w-11;
  mobile drawer links min-h-11 px-3 rounded-md. WCAG 2.5.5 tap
  target 44x44 minimum.
- install-command.tsx: copy button h-8 w-8 → h-11 w-11 (was 32px,
  below WCAG). Icon stays 16x16; padding fills to 44.
- middleware.ts: matcher excludes opengraph-image, twitter-image,
  icon, apple-icon, manifest.webmanifest, robots.txt, sitemap.xml.
  Previously next-intl redirected /opengraph-image → /en/opengraph-image
  breaking OG previews for crawlers that hit the unlocalized path.

packages/web/components/footer.tsx

@@ -1,7 +1,8 @@
+import type { JSX } from "react"
 import { getTranslations } from "next-intl/server"
 import { Link } from "@/i18n/routing"
 
-export async function Footer({ locale }: { locale?: string } = {}) {
+export async function Footer({ locale }: { readonly locale?: string } = {}): Promise<JSX.Element> {
   const t = locale
     ? await getTranslations({ locale, namespace: "footer" })
     : await getTranslations("footer")
@@ -27,7 +28,7 @@ export async function Footer({ locale }: { locale?: string } = {}) {
               {t("github")}
             </a>
             <a
-              href="https://discord.gg/indentcorp"
+              href="https://discord.gg/PUwSMR9XNk"
               target="_blank"
               rel="noopener noreferrer"
               className="transition-colors hover:text-cyan-400"