add brand/ assets to repo + BRAND.md as single source of truth

BRAND.md: Complete brand guide with code snippets, color tokens, typography scale,
logo usage rules, DO/DON'T checklist, photography direction, voice & tone.
Every section references its visual asset in brand/.

brand/ folder (52 assets):
- photography/ (20) — landing page photos
- logo/ (6) — lockup, reversed, blue, marks, favicon
- color-palette/ (3) — primary, tints, psychology
- typography/ (3) — specimen, scale, numbers
- moodboard/ (3) — trust, community, editorial
- brand-guide/ (7) — cover through UI patterns
- social-templates/ (4) — OG, Instagram, Story, LinkedIn
- icons/ (6) — pledge, whatsapp, gift-aid, zakat, dashboard, schedule

Also fixed: Nav component still had backdrop-blur-lg (last violation)

pledge-now-pay-later/src/app/for/_components/index.tsx

@@ -4,7 +4,7 @@ import Image from "next/image"
 /* ── Nav ── */
 export function Nav() {
   return (
-    <header className="sticky top-0 z-40 border-b border-gray-100 bg-white/90 backdrop-blur-lg">
+    <header className="sticky top-0 z-40 border-b border-gray-200 bg-white">
       <div className="max-w-5xl mx-auto flex h-14 items-center justify-between px-6">
         <Link href="/" className="flex items-center gap-2.5">
           <div className="h-7 w-7 bg-gray-900 flex items-center justify-center">