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>

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2015-present 650 Industries, Inc. (aka Expo)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
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