feat: public landing page with magic login, brand palette and demo build flag

Adds a public funnel landing at `/` and moves the login to `/login`, rebuilt
around the panel's own colors instead of an invented palette.

Landing (`src/components/landing/`, one section per file, no props):
- Seven funnel sections composed by `LandingPage`. The hero's eight swatches are
  literally `PIE_COLORS` from `DashboardPage`, the same hex values the seed hands
  to avatars and services, so "your colors become your numbers" is literal.
- `BrandMark` becomes the single source for the logo, replicating
  `public/favicon.svg`. Blue is the action surface, orange only ever marks.
- `NotebookVisual` is the one deliberate exception to the palette: it is what the
  product replaces.
- Copy drops all system vocabulary; motion comes from `src/lib/motion.ts` with a
  single easing, and reduced-motion resolves `initial` to the final state so a
  never-firing `whileInView` cannot leave a section invisible forever.

Routing and bundle:
- `homePathFor` is the single definition of each role's destination.
- Landing, login, dashboard and calendar load lazily. Eager, the login dragged
  framer-motion (~40 KB gz) into every panel load and the landing downloaded
  recharts + FullCalendar (~187 KB gz) without charting anything. `clsx` is
  pinned to the `react` chunk because Rollup otherwise assigns it to `charts`,
  making the entry import 111 KB gz for a 200-byte utility.

Responsiveness (iPhone/iPad), verified with `npm run audit:responsive`:
- No touch form field below 16px, `dvh` height utilities, safe-area insets, and
  40px touch targets keyed off `pointer: coarse` rather than `sm:`.
- New `.ld-gutter`: `.safe-x` lives outside `@layer` and beats Tailwind's `px-*`,
  so it left the login's side padding at 0 on anything but an iPhone in
  landscape. No overflow check could see it — there was no overflow, just zero
  margin. The audit now guards it with a `gutter` check.
- `shell-height` no longer fires on pages that legitimately scroll; the static
  `raw-viewport-unit` scan covers those instead.

Production build:
- The Dockerfile now sets `VITE_DEMO_UI=1` as a build arg. `DEMO` is a build-time
  constant, so without it Vite eliminated the magic login and the "Ver como…"
  switcher: the deployed landing promised "no registration" and led to an empty
  form. Verified by building both ways and diffing the bundle.
- Service worker cache bumped to v2 so the orphaned pre-landing chunks get purged
  from returning visitors' caches.

Verified: typecheck clean; unit 39/39, e2e 33/33, admin 17/17, booking 12/12,
landing 13/13 (WebKit), PWA passed against the real production build.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/components/landing/LandingNav.tsx

@@ -0,0 +1,45 @@
+import { Link } from "react-router-dom";
+import { motion, useScroll, useTransform } from "framer-motion";
+import { useReducedMotion } from "../../lib/useReducedMotion";
+import { BrandMark } from "../BrandMark";
+
+/**
+ * Nav flotante. Arranca casi transparente sobre el hero y se condensa al bajar:
+ * fondo y desenfoque suben juntos, que es lo que evita el salto brusco de un
+ * `position: fixed` con fondo sólido desde el primer píxel.
+ */
+export function LandingNav() {
+  const reduced = useReducedMotion();
+  const { scrollY } = useScroll();
+  const fondo = useTransform(scrollY, [0, 140], ["rgba(255,251,247,0)", "rgba(255,251,247,0.86)"]);
+  const desenfoque = useTransform(scrollY, [0, 140], ["blur(0px)", "blur(16px)"]);
+  const borde = useTransform(scrollY, [0, 140], ["rgba(27,21,51,0)", "rgba(27,21,51,0.1)"]);
+
+  return (
+    <motion.header
+      className="safe-top fixed inset-x-0 top-0 z-50 border-b"
+      style={
+        reduced
+          ? { background: "rgba(255,251,247,0.92)", borderColor: "rgba(27,21,51,0.1)" }
+          : { background: fondo, backdropFilter: desenfoque, borderColor: borde }
+      }
+    >
+      <nav className="ld-wrap flex items-center justify-between gap-4 px-5 py-3">
+        <Link
+          to="/"
+          className="tap-target flex items-center gap-2.5 text-[1.05rem] font-extrabold tracking-tight"
+        >
+          <BrandMark className="h-8 w-8" />
+          AgendaMax
+        </Link>
+        <Link
+          to="/login"
+          className="ld-cta"
+          style={{ minHeight: 46, paddingInline: "1.35rem", fontSize: "0.9rem" }}
+        >
+          Entrar
+        </Link>
+      </nav>
+    </motion.header>
+  );
+}