feat: overhaul TUI dashboard with subscription data, new tabs, and setup wizard

Major rewrite of the Bubble Tea dashboard, adding live claude.ai
integration and splitting the monolithic app.go into focused tab modules.

App model (app.go):
- Integrate claudeai.Client for live subscription/rate-limit data
- Add SubDataMsg and async fetch with periodic refresh (every 5 min)
- Add spinner for loading states (charmbracelet/bubbles spinner)
- Integrate huh form library for in-TUI setup wizard
- Rework tab routing to dispatch to dedicated tab renderers
- Add compact layout detection for narrow terminals (<100 cols)

TUI setup wizard (setup.go):
- Full huh-based setup flow embedded in the TUI (not just CLI)
- Three-step form: credentials, preferences (time range + theme), confirm
- Pre-populates from existing config, validates session key prefix
- Returns to dashboard on completion with config auto-saved

New tab modules:
- tab_overview.go: summary cards (sessions, prompts, cost, time), daily
  activity sparkline, rate-limit progress bars from live subscription data
- tab_breakdown.go: per-model usage table with calls, input/output tokens,
  cost, and share percentage; compact mode for narrow terminals
- tab_costs.go: cost analysis with daily cost chart, model cost breakdown,
  cache efficiency metrics, and budget tracking with progress bar

Rewritten tabs:
- tab_sessions.go: paginated session browser with sort-by-cost/tokens/time,
  per-session detail view, model usage breakdown per session, improved
  navigation (j/k, enter/esc, n/p for pages)
- tab_settings.go: updated to work with new theme struct and config fields

internal/tui/tab_settings.go

@@ -2,6 +2,7 @@ package tui
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 
 	"cburn/internal/cli"
@@ -16,6 +17,7 @@ import (
 
 const (
 	settingsFieldAPIKey = iota
+	settingsFieldSessionKey
 	settingsFieldTheme
 	settingsFieldDays
 	settingsFieldBudget
@@ -56,13 +58,21 @@ func (a App) settingsStartEdit() (tea.Model, tea.Cmd) {
 		if existing != "" {
 			ti.SetValue(existing)
 		}
+	case settingsFieldSessionKey:
+		ti.Placeholder = "sk-ant-sid..."
+		ti.EchoMode = textinput.EchoPassword
+		ti.EchoCharacter = '*'
+		existing := config.GetSessionKey(cfg)
+		if existing != "" {
+			ti.SetValue(existing)
+		}
 	case settingsFieldTheme:
 		ti.Placeholder = "flexoki-dark, catppuccin-mocha, tokyo-night, terminal"
 		ti.SetValue(cfg.Appearance.Theme)
 		ti.EchoMode = textinput.EchoNormal
 	case settingsFieldDays:
 		ti.Placeholder = "30"
-		ti.SetValue(fmt.Sprintf("%d", cfg.General.DefaultDays))
+		ti.SetValue(strconv.Itoa(cfg.General.DefaultDays))
 		ti.EchoMode = textinput.EchoNormal
 	case settingsFieldBudget:
 		ti.Placeholder = "500 (monthly USD, leave empty to clear)"
@@ -103,6 +113,8 @@ func (a *App) settingsSave() {
 	switch a.settings.cursor {
 	case settingsFieldAPIKey:
 		cfg.AdminAPI.APIKey = val
+	case settingsFieldSessionKey:
+		cfg.ClaudeAI.SessionKey = val
 	case settingsFieldTheme:
 		// Validate theme name
 		found := false
@@ -162,10 +174,21 @@ func (a App) renderSettingsTab(cw int) string {
 		}
 	}
 
+	sessionKeyDisplay := "(not set)"
+	existingSession := config.GetSessionKey(cfg)
+	if existingSession != "" {
+		if len(existingSession) > 16 {
+			sessionKeyDisplay = existingSession[:12] + "..." + existingSession[len(existingSession)-4:]
+		} else {
+			sessionKeyDisplay = "****"
+		}
+	}
+
 	fields := []field{
 		{"Admin API Key", apiKeyDisplay},
+		{"Session Key", sessionKeyDisplay},
 		{"Theme", cfg.Appearance.Theme},
-		{"Default Days", fmt.Sprintf("%d", cfg.General.DefaultDays)},
+		{"Default Days", strconv.Itoa(cfg.General.DefaultDays)},
 		{"Monthly Budget", func() string {
 			if cfg.Budget.MonthlyUSD != nil {
 				return fmt.Sprintf("$%.0f", *cfg.Budget.MonthlyUSD)
@@ -210,7 +233,7 @@ func (a App) renderSettingsTab(cw int) string {
 	infoBody.WriteString(labelStyle.Render("Data directory:  ") + valueStyle.Render(a.claudeDir) + "\n")
 	infoBody.WriteString(labelStyle.Render("Sessions loaded: ") + valueStyle.Render(cli.FormatNumber(int64(len(a.sessions)))) + "\n")
 	infoBody.WriteString(labelStyle.Render("Load time:       ") + valueStyle.Render(fmt.Sprintf("%.1fs", a.loadTime.Seconds())) + "\n")
-	infoBody.WriteString(labelStyle.Render("Config file:     ") + valueStyle.Render(config.ConfigPath()))
+	infoBody.WriteString(labelStyle.Render("Config file:     ") + valueStyle.Render(config.Path()))
 
 	var b strings.Builder
 	b.WriteString(components.ContentCard("Settings", formBody.String(), cw))