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.json

@@ -0,0 +1,32 @@
+{
+  "expo": {
+    "name": "Dokio",
+    "slug": "dokio",
+    "scheme": "dokio",
+    "version": "1.0.0",
+    "orientation": "portrait",
+    "icon": "./assets/icon.png",
+    "userInterfaceStyle": "light",
+    "ios": {
+      "supportsTablet": true
+    },
+    "android": {
+      "adaptiveIcon": {
+        "backgroundColor": "#EEF3FC",
+        "foregroundImage": "./assets/android-icon-foreground.png",
+        "backgroundImage": "./assets/android-icon-background.png",
+        "monochromeImage": "./assets/android-icon-monochrome.png"
+      },
+      "predictiveBackGestureEnabled": false
+    },
+    "web": {
+      "bundler": "metro",
+      "output": "single",
+      "favicon": "./assets/favicon.png"
+    },
+    "plugins": [
+      "expo-router",
+      "expo-font"
+    ]
+  }
+}