feat(web): mobile-responsive /templates + drop pre-launch SiteBanner

Two related polish items:

1. Remove the global blue Preview banner from app/layout.tsx and delete
   the SiteBanner component. The component's own comment said "Remove
   once the service is open for production use" — Stripe live billing,
   OAuth, and per-runner TLS are all wired now, so the pre-launch notice
   is misleading.

2. Mobile-responsive treatment for the standalone /templates page (it
   lives outside (dashboard) layout, so it didn't inherit the new
   mobile chrome from the dashboard pass):
   - Top header tightened: "/templates" breadcrumb + Dashboard link +
     "+ New server" pill all hidden on mobile (the avatar UserMenu +
     bottom MobileActionBar cover those paths).
   - Logged-in users now get the same MobileActionBar tab-bar at the
     bottom (Market tab active), giving consistent app-shell across
     dashboard pages.
   - Filter row stacks vertically on mobile with search on top (thumb
     reach), then a horizontally-scrollable chip row for scope / sort /
     category so segmented controls don't squeeze below their min-width.
   - h1 scales 32px → 24px on mobile; padding tightened to px-4 py-8.
   - main gets pb-24 when logged in so cards clear the tab bar.

Logged-out marketplace browsing keeps the simpler marketing chrome
(Logo + "Start building" CTA) — no tab-bar, since visitors don't have
a dashboard to navigate into yet.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/components/site-banner.tsx

@@ -1,17 +0,0 @@
-// Temporary pre-launch notice. Remove this component and its import in
-// app/layout.tsx once the service is open for production use.
-// Background is #4f46e5 (not the #6366f1 accent) so white text clears the
-// WCAG AA 4.5:1 contrast ratio for this small text — #6366f1 fell just short.
-export function SiteBanner() {
-  return (
-    <div
-      style={{ backgroundColor: '#4f46e5' }}
-      className="border-b border-[#4338ca] px-4 py-2 text-center text-[12.5px] font-medium leading-snug text-white"
-    >
-      <span className="font-semibold uppercase tracking-wide">Preview</span>
-      <span className="px-1.5">·</span>
-      BuildMyMCPServer is not yet open for production use — sign-ups, server generation and billing
-      are still being finalised, and data may be reset.
-    </div>
-  );
-}