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/lib/useBreakpoint.ts

@@ -0,0 +1,45 @@
+import { useEffect, useState } from "react";
+
+/**
+ * Tres tamaños, alineados con los breakpoints del CSS del calendario:
+ *   phone   <= 640px    (iPhone 12: 390, iPhone 16 Pro Max: 440)
+ *   tablet  641-1024px  (iPad mini: 744, iPad 10.9": 820, iPad Pro portrait: 1024)
+ *   desktop >= 1025px   (iPad landscape: 1180, iPad Pro landscape: 1366)
+ *
+ * El límite de tablet incluye 1024 a propósito: un iPad Pro en portrait mide justo
+ * 1024px y, descontando la barra lateral, deja ~712px para el calendario. Con 7
+ * columnas son ~100px por día, y con dos citas solapadas los títulos quedan en
+ * "A...". No coincide con el `lg:` de Tailwind (también 1024), así que a 1024px hay
+ * barra lateral fija Y vista de 3 días: es justo lo que se busca en ese equipo.
+ */
+export type Breakpoint = "phone" | "tablet" | "desktop";
+
+const PHONE = "(max-width: 640px)";
+const TABLET = "(min-width: 641px) and (max-width: 1024px)";
+
+function currentBreakpoint(): Breakpoint {
+  if (typeof window === "undefined") return "desktop";
+  if (window.matchMedia(PHONE).matches) return "phone";
+  if (window.matchMedia(TABLET).matches) return "tablet";
+  return "desktop";
+}
+
+export function useBreakpoint(): Breakpoint {
+  const [bp, setBp] = useState<Breakpoint>(currentBreakpoint);
+
+  useEffect(() => {
+    const lists = [window.matchMedia(PHONE), window.matchMedia(TABLET)];
+    const onChange = () => setBp(currentBreakpoint());
+    lists.forEach((l) => l.addEventListener("change", onChange));
+    // Al rotar un iPad se cruza de 820 a 1180 y los media queries ya lo cubren,
+    // pero en iOS el evento de orientación llega antes de que se reasienten las
+    // medidas; re-evaluamos para no quedar con el layout de la orientación previa.
+    window.addEventListener("orientationchange", onChange);
+    return () => {
+      lists.forEach((l) => l.removeEventListener("change", onChange));
+      window.removeEventListener("orientationchange", onChange);
+    };
+  }, []);
+
+  return bp;
+}