Implement Dokio screens from the design doc (turns 3 + 4)

Scaffold an Expo + TypeScript app for Dokio, the app that scans
Behördenpost and explains it in plain language, and build the four
screens specified in "Dokio Design.dc.html":

  4a  Onboarding, full-bleed dither gradient
  4b  Home, dithered scan card
  3a  Kamera, mocked viewfinder
  3b  Archiv, search + filters + month groups

Design tokens from the turn 4 blue palette live in src/theme.ts.
Content is mocked in src/data/letters.ts, taken verbatim from the doc.

DitherBackground is a GLSL port of the doc's dither-bg.js, which drew a
low-resolution canvas and upscaled it with image-rendering: pixelated;
the fragment shader snaps to the same grid instead. Two notes on the
port:

- The original's fract(sin(x) * 43758.5453) hash only distributes well
  in float64. A fragment shader's highp float has ~24 mantissa bits, so
  it collapsed to a near-constant field and the gradient bands rendered
  straight rather than warped. Replaced with Hoskins' hash, verified
  against the original running side by side.
- The 4x4 Bayer matrix is built recursively rather than as a lookup,
  which avoids dynamic array indexing; the construction reproduces all
  16 entries of the original exactly.

The camera screen deliberately stops short of expo-camera: the
viewfinder is a static stand-in, so the flow is navigable without
permissions. The 2a result screen is not part of turns 3 and 4 and is
not included.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/_layout.tsx

@@ -0,0 +1,42 @@
+import { BricolageGrotesque_800ExtraBold } from '@expo-google-fonts/bricolage-grotesque';
+import {
+  NunitoSans_400Regular,
+  NunitoSans_600SemiBold,
+  NunitoSans_700Bold,
+  NunitoSans_800ExtraBold,
+} from '@expo-google-fonts/nunito-sans';
+import { useFonts } from 'expo-font';
+import { Stack } from 'expo-router';
+import { StatusBar } from 'expo-status-bar';
+import { ActivityIndicator, View } from 'react-native';
+import { SafeAreaProvider } from 'react-native-safe-area-context';
+import { colors } from '../src/theme';
+
+export default function RootLayout() {
+  const [fontsLoaded] = useFonts({
+    BricolageGrotesque_800ExtraBold,
+    NunitoSans_400Regular,
+    NunitoSans_600SemiBold,
+    NunitoSans_700Bold,
+    NunitoSans_800ExtraBold,
+  });
+
+  if (!fontsLoaded) {
+    return (
+      <View style={{ flex: 1, backgroundColor: colors.canvas, justifyContent: 'center' }}>
+        <ActivityIndicator color={colors.blue} />
+      </View>
+    );
+  }
+
+  return (
+    <SafeAreaProvider>
+      <StatusBar style="auto" />
+      <Stack screenOptions={{ headerShown: false, contentStyle: { backgroundColor: colors.canvas } }}>
+        <Stack.Screen name="onboarding" />
+        <Stack.Screen name="(tabs)" />
+        <Stack.Screen name="scan" options={{ presentation: 'fullScreenModal', animation: 'fade' }} />
+      </Stack>
+    </SafeAreaProvider>
+  );
+}