Each screen is now a self-contained `page` block with its own `enter`,
`render`, and `key` handlers. Shared logic lives in a `helpers` block
mixed into the render context. A `chrome` block renders persistent UI
(header, sidebar, footer) before every frame.
New context primitives: `go(page)` for transitions, `reload` to re-run
the current page's `enter` block, `printable` key binding for text input.
`FlowRunner#run_tui` now passes the flow context to TUIRunner so username
and other session data are available without reaching into internals.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
