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/(auth)/layout.tsx

@@ -1,18 +1,12 @@
 import Link from "next/link";
-import { Mic } from "lucide-react";
+import { Logo } from "@/components/ui/logo";
 
 export default function AuthLayout({ children }: { children: React.ReactNode }) {
   return (
     <div className="relative flex min-h-screen flex-col items-center justify-center bg-secondary px-4 py-12">
       <div className="pointer-events-none absolute inset-0 bg-hero-wash" />
-      <Link
-        href="/"
-        className="relative mb-8 flex items-center gap-2.5 font-display text-xl font-bold tracking-tight"
-      >
-        <span className="flex h-9 w-9 items-center justify-center rounded-2xl bg-brand text-brand-foreground">
-          <Mic className="h-5 w-5" />
-        </span>
-        Podcast Distribution AI
+      <Link href="/" className="relative mb-8 flex items-center" aria-label="Podcast Distribution AI">
+        <Logo className="h-8 w-auto" />
       </Link>
       <div className="relative w-full max-w-md">{children}</div>
     </div>