feat: add TUI feature modules for setup wizard, session browser, and settings

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.

internal/tui/setup.go

@@ -0,0 +1,152 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+
+	"cburn/internal/config"
+	"cburn/internal/tui/theme"
+
+	"github.com/charmbracelet/bubbles/textinput"
+	"github.com/charmbracelet/lipgloss"
+)
+
+// setupState tracks the first-run setup wizard state.
+type setupState struct {
+	active    bool
+	step      int // 0=welcome, 1=api key, 2=days, 3=theme, 4=done
+	apiKeyIn  textinput.Model
+	daysChoice int   // index into daysOptions
+	themeChoice int  // index into theme.All
+	saveErr    error // non-nil if config save failed
+}
+
+var daysOptions = []struct {
+	label string
+	value int
+}{
+	{"7 days", 7},
+	{"30 days", 30},
+	{"90 days", 90},
+}
+
+func newSetupState() setupState {
+	ti := textinput.New()
+	ti.Placeholder = "sk-ant-admin-... (or press Enter to skip)"
+	ti.CharLimit = 256
+	ti.Width = 50
+	ti.EchoMode = textinput.EchoPassword
+	ti.EchoCharacter = '*'
+
+	return setupState{
+		apiKeyIn:   ti,
+		daysChoice: 1, // default 30 days
+	}
+}
+
+func (a App) renderSetup() string {
+	t := theme.Active
+	ss := a.setup
+
+	titleStyle := lipgloss.NewStyle().Foreground(t.Accent).Bold(true)
+	labelStyle := lipgloss.NewStyle().Foreground(t.TextMuted)
+	valueStyle := lipgloss.NewStyle().Foreground(t.TextPrimary)
+	accentStyle := lipgloss.NewStyle().Foreground(t.Accent)
+	greenStyle := lipgloss.NewStyle().Foreground(t.Green)
+
+	var b strings.Builder
+	b.WriteString("\n\n")
+	b.WriteString(titleStyle.Render("  Welcome to cburn!"))
+	b.WriteString("\n\n")
+
+	b.WriteString(labelStyle.Render(fmt.Sprintf("  Found %s sessions in %s",
+		valueStyle.Render(fmt.Sprintf("%d", len(a.sessions))),
+		valueStyle.Render(a.claudeDir))))
+	b.WriteString("\n\n")
+
+	switch ss.step {
+	case 0: // Welcome
+		b.WriteString(valueStyle.Render("  Let's set up a few things."))
+		b.WriteString("\n\n")
+		b.WriteString(accentStyle.Render("  Press Enter to continue"))
+
+	case 1: // API key
+		b.WriteString(valueStyle.Render("  1. Anthropic Admin API key"))
+		b.WriteString("\n")
+		b.WriteString(labelStyle.Render("     For real cost data from the billing API."))
+		b.WriteString("\n")
+		b.WriteString(labelStyle.Render("     Get one at console.anthropic.com > Settings > Admin API keys"))
+		b.WriteString("\n\n")
+		b.WriteString("     ")
+		b.WriteString(ss.apiKeyIn.View())
+		b.WriteString("\n\n")
+		b.WriteString(labelStyle.Render("     Press Enter to continue (leave blank to skip)"))
+
+	case 2: // Default days
+		b.WriteString(valueStyle.Render("  2. Default time range"))
+		b.WriteString("\n\n")
+		for i, opt := range daysOptions {
+			if i == ss.daysChoice {
+				b.WriteString(accentStyle.Render(fmt.Sprintf("     (o) %s", opt.label)))
+			} else {
+				b.WriteString(labelStyle.Render(fmt.Sprintf("     ( ) %s", opt.label)))
+			}
+			b.WriteString("\n")
+		}
+		b.WriteString("\n")
+		b.WriteString(labelStyle.Render("     j/k to select, Enter to confirm"))
+
+	case 3: // Theme
+		b.WriteString(valueStyle.Render("  3. Color theme"))
+		b.WriteString("\n\n")
+		for i, th := range theme.All {
+			if i == ss.themeChoice {
+				b.WriteString(accentStyle.Render(fmt.Sprintf("     (o) %s", th.Name)))
+			} else {
+				b.WriteString(labelStyle.Render(fmt.Sprintf("     ( ) %s", th.Name)))
+			}
+			b.WriteString("\n")
+		}
+		b.WriteString("\n")
+		b.WriteString(labelStyle.Render("     j/k to select, Enter to confirm"))
+
+	case 4: // Done
+		if ss.saveErr != nil {
+			warnStyle := lipgloss.NewStyle().Foreground(t.Orange)
+			b.WriteString(warnStyle.Render(fmt.Sprintf("  Could not save config: %s", ss.saveErr)))
+			b.WriteString("\n")
+			b.WriteString(labelStyle.Render("  Settings will apply for this session only."))
+		} else {
+			b.WriteString(greenStyle.Render("  All set!"))
+			b.WriteString("\n\n")
+			b.WriteString(labelStyle.Render("  Saved to ~/.config/cburn/config.toml"))
+			b.WriteString("\n")
+			b.WriteString(labelStyle.Render("  Run `cburn setup` anytime to reconfigure."))
+		}
+		b.WriteString("\n\n")
+		b.WriteString(accentStyle.Render("  Press Enter to launch the dashboard"))
+	}
+
+	return b.String()
+}
+
+func (a *App) saveSetupConfig() {
+	cfg, _ := config.Load()
+
+	apiKey := strings.TrimSpace(a.setup.apiKeyIn.Value())
+	if apiKey != "" {
+		cfg.AdminAPI.APIKey = apiKey
+	}
+
+	if a.setup.daysChoice >= 0 && a.setup.daysChoice < len(daysOptions) {
+		cfg.General.DefaultDays = daysOptions[a.setup.daysChoice].value
+		a.days = cfg.General.DefaultDays
+	}
+
+	if a.setup.themeChoice >= 0 && a.setup.themeChoice < len(theme.All) {
+		cfg.Appearance.Theme = theme.All[a.setup.themeChoice].Name
+		theme.SetActive(cfg.Appearance.Theme)
+	}
+
+	a.setup.saveErr = config.Save(cfg)
+}