fix(leads): portaled dropdowns, flex-wrapper icons, responsive layout, and UI polish

- Replace all native <select> with LeadCustomSelect (ReactDOM.createPortal into body,
  getBoundingClientRect positioning, e.composedPath() scroll detection)
- Add LeadCombobox: fuzzy-search state picker (searches abbr + full name, portal, onMouseDown)
- Convert Phone/MapPin icons from absolute-positioned to flex-wrapper siblings of bg-transparent input
- Fix dark mode invisible text: all inputs converted from isDark JS conditionals to dark: variants
- Fix urgency Standard pill visibility: container bg-zinc-900, selected bg-zinc-700 ring-1 shadow-lg
- Fix City/State/ZIP mobile overlap: flex -> grid-cols-2 (mobile) / grid-cols-[1fr_5.5rem_6rem] (sm+)
- Fix urgency section hidden behind mobile sticky footer: pb-28 -> pb-44 md:pb-10
- Replace all compact type pickers (phone/email) with LeadCustomSelect compact mode
- Update README.md with full lead creation architecture docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/CreateLeadPage.jsx

@@ -245,7 +245,7 @@ export default function CreateLeadPage() {
             {/* ----------------------------------------------------------------
                 Page content
             ---------------------------------------------------------------- */}
-            <div className="max-w-3xl mx-auto px-4 sm:px-6 py-6 lg:py-10 pb-28 md:pb-10">
+            <div className="max-w-3xl mx-auto px-4 sm:px-6 py-6 lg:py-10 pb-44 md:pb-10">
 
                 {/* --- Page Header --- */}
                 <div className="mb-7">