Add brand logo + favicon across app surfaces

Ship the Podcast Distribution AI wordmark as real assets and replace the
Mic-tile + text placeholder everywhere it appeared.

- public/logo-dark.png (dark wordmark, for light backgrounds) and
  public/logo-light.png (light wordmark, for dark backgrounds)
- New <Logo> component swaps the two via Tailwind dark: variants, so the
  dark logo shows on all non-themed surfaces (marketing, auth, admin,
  public share) and the light logo only in dark-mode app surfaces
- Wire <Logo> into the marketing header/footer, auth layout, app header
  (default branch only - white-label org logos untouched), admin header
  (+ Admin badge), and the public share page
- Favicon via App Router file convention: app/icon.png + app/apple-icon.png

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/(admin)/layout.tsx

@@ -1,10 +1,11 @@
 import Link from "next/link";
-import { ShieldCheck, ArrowLeft } from "lucide-react";
+import { ArrowLeft } from "lucide-react";
 import { requireAdmin } from "@/lib/auth/guards";
 import { AdminSidebar } from "@/components/admin/admin-sidebar";
 import { AdminMobileNav } from "@/components/admin/admin-mobile-nav";
 import { UserMenu } from "@/components/app/user-menu";
 import { Button } from "@/components/ui/button";
+import { Logo } from "@/components/ui/logo";
 
 // Authed, DB-backed admin surface — never statically prerender.
 export const dynamic = "force-dynamic";
@@ -17,11 +18,11 @@ export default async function AdminLayout({ children }: { children: React.ReactN
       <header className="sticky top-0 z-30 flex h-16 items-center justify-between border-b border-border bg-background/85 px-4 backdrop-blur-md md:px-6">
         <div className="flex items-center gap-1">
           <AdminMobileNav />
-          <Link href="/admin" className="flex items-center gap-2.5 font-display font-bold tracking-tight">
-            <span className="flex h-9 w-9 items-center justify-center rounded-2xl bg-foreground text-background">
-              <ShieldCheck className="h-5 w-5" />
+          <Link href="/admin" className="flex items-center gap-2.5" aria-label="Podcast Distribution AI Admin">
+            <Logo className="h-7 w-auto" priority />
+            <span className="rounded-md bg-foreground px-1.5 py-0.5 font-display text-[11px] font-bold uppercase tracking-wide text-background">
+              Admin
             </span>
-            <span>Podcast Distribution AI Admin</span>
           </Link>
         </div>
         <div className="flex items-center gap-3">