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  (was outdated invite that 404'd).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/landing/install-command.tsx

@@ -27,7 +27,7 @@ export function InstallCommand({ command }: { command: string }) {
         <Button
           variant="ghost"
           size="icon"
-          className="h-8 w-8 text-zinc-400 hover:text-white"
+          className="h-11 w-11 text-zinc-400 hover:text-white"
           onClick={copyCommand}
           aria-label="Copy install command"
         >