Prepare product layout and reusable Flutter theme

Relocate the existing Astro website and preserve its assets and hosting configuration. Add the consumer/backend scaffold and themed Flutter package with consumer tests. Related to #1.

website/src/lib/app-links.ts

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+// Full Flutter destinations are intentionally unset until the real app exists.
+function validatedUrl(value: string | undefined) {
+  if (!value) return undefined;
+  const url = new URL(value);
+  if (!['https:', 'http:'].includes(url.protocol))
+    throw new Error('Flutter URLs must be HTTP(S) URLs');
+  return url.toString();
+}
+export const appLinks = {
+  register: validatedUrl(import.meta.env.PUBLIC_FLUTTER_REGISTER_URL),
+  login: validatedUrl(import.meta.env.PUBLIC_FLUTTER_LOGIN_URL),
+};

website/src/lib/quote.ts

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+export const cryptoCurrencies = ['BTC', 'ETH', 'USDT'] as const;
+export const fiatCurrencies = ['EUR', 'USD'] as const;
+export type Crypto = (typeof cryptoCurrencies)[number];
+export type Fiat = (typeof fiatCurrencies)[number];
+// Fixed, deliberately illustrative fixtures. Never use for an actual exchange.
+export const demoRates: Record<Crypto, Record<Fiat, number>> = {
+  BTC: { EUR: 57500, USD: 62500 },
+  ETH: { EUR: 2300, USD: 2500 },
+  USDT: { EUR: 0.92, USD: 1 },
+};
+export function parseAmount(raw: string): number {
+  const normalized = raw.trim().replace(',', '.');
+  if (!/^\d+(?:\.\d{0,8})?$/.test(normalized)) return NaN;
+  return Number(normalized);
+}
+export function calculateQuote(amount: number, crypto: Crypto, fiat: Fiat) {
+  if (!Number.isFinite(amount) || amount <= 0 || amount > 1_000_000)
+    throw new RangeError('Invalid amount');
+  if (!cryptoCurrencies.includes(crypto) || !fiatCurrencies.includes(fiat))
+    throw new RangeError('Invalid currency');
+  const rate = demoRates[crypto][fiat];
+  const grossCents = Math.round(amount * rate * 100);
+  const feeCents = Math.round(grossCents * 0.01);
+  return {
+    rate,
+    fee: feeCents / 100,
+    receive: (grossCents - feeCents) / 100,
+    demo: true,
+  };
+}