Add three self-contained feature modules that plug into the root App
model via shared state structs and render methods.
setup.go -- First-run wizard with a 5-step flow: welcome screen, API
key entry (password-masked text input), default time range selector
(radio-style j/k navigation over 7/30/90 day options), theme picker
(radio-style over all registered themes), and a completion screen
that persists choices to ~/.config/cburn/config.toml. Gracefully
handles save failures by noting the settings apply for the current
session only.
tab_sessions.go -- Session browser with two view modes: split view
(1/3 condensed list + 2/3 detail pane, scrollable with offset
tracking) and full-screen detail. The detail body shows duration,
prompt/API-call ratio, per-type token breakdown with cache cost
attribution, per-model API call table, and subagent indicator.
tab_settings.go -- Runtime settings editor with 4 configurable
fields (API key, theme, default days, monthly budget). Supports
inline text input editing with Enter/Esc save/cancel flow, immediate
config persistence, flash "Saved!" confirmation, and error display
on save failure. Theme changes apply instantly without restart.
