Add tidy, an optional AI helper, screen descriptions and a hand-editable prompt

Tidy (lib/tidy.ts) lays out one phone screen by rule: neighbouring list items
and buttons join into connected runs, bars snap to the edges, the FAB to the
corner, dialogs to the centre, and the rest stacks on 16dp margins without
running under the bottom bars. The button sits beside the zoom pill and in the
screen inspector, turns into "undo" right after, and is disabled once tidy.

The AI helper (lib/ai.ts, the AI tab) calls OpenAI, Claude, Gemini or DeepSeek
straight from the browser with the author's own key, kept in localStorage.
Each behavior note and screen description gets an "AI" button that writes the
field in the UI language and an undo that swaps the text back. While a request
runs the screen's bezel shows a drifting gradient of the scheme's colors.

Screens gain a description that goes into the prompt, and the whole prompt can
be edited by hand before copying. README and SECURITY.md describe what the AI
helper sends and where the key lives.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>