feat: add TUI auto-refresh with configurable interval and manual refresh

Introduce background data refresh so the dashboard stays current without
restarting. This touches four layers:

Config (config.go):
- New TUIConfig struct with AutoRefresh (bool) and RefreshIntervalSec
  (int) fields, defaulting to enabled at 30-second intervals.
- Minimum interval floor of 10 seconds enforced at load time.

App core (app.go):
- RefreshDataMsg type for background refresh completion signaling.
- Auto-refresh state: interval timer, refreshing flag, lastRefresh
  timestamp. Checked on every tick; fires refreshDataCmd when elapsed.
- refreshDataCmd: background goroutine that loads session data via cache
  (with uncached fallback) and posts RefreshDataMsg on completion.
- Manual refresh keybind: 'r' triggers immediate refresh.
- Auto-refresh toggle keybind: 'R' toggles auto-refresh and persists
  the preference to config.toml.
- Help text updated with r/R keybind documentation.

Status bar (statusbar.go):
- Shows spinning refresh indicator during active refresh.
- Shows auto-refresh icon when auto-refresh is enabled.

Settings tab (tab_settings.go):
- Two new editable fields: Auto Refresh (bool) and Refresh Interval
  (seconds with 10s minimum).
- Settings display reads live App state to stay consistent with the
  R toggle keybind (avoids stale config-file reads).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/tui/tab_settings.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+	"time"
 
 	"cburn/internal/cli"
 	"cburn/internal/config"
@@ -21,6 +22,8 @@ const (
 	settingsFieldTheme
 	settingsFieldDays
 	settingsFieldBudget
+	settingsFieldAutoRefresh
+	settingsFieldRefreshInterval
 	settingsFieldCount // sentinel
 )
 
@@ -80,6 +83,19 @@ func (a App) settingsStartEdit() (tea.Model, tea.Cmd) {
 			ti.SetValue(fmt.Sprintf("%.0f", *cfg.Budget.MonthlyUSD))
 		}
 		ti.EchoMode = textinput.EchoNormal
+	case settingsFieldAutoRefresh:
+		ti.Placeholder = "true or false"
+		ti.SetValue(strconv.FormatBool(a.autoRefresh))
+		ti.EchoMode = textinput.EchoNormal
+	case settingsFieldRefreshInterval:
+		ti.Placeholder = "30 (seconds, minimum 10)"
+		// Use effective value from App state to match display
+		intervalSec := int(a.refreshInterval.Seconds())
+		if intervalSec < 10 {
+			intervalSec = 30
+		}
+		ti.SetValue(strconv.Itoa(intervalSec))
+		ti.EchoMode = textinput.EchoNormal
 	}
 
 	ti.Focus()
@@ -144,6 +160,15 @@ func (a *App) settingsSave() {
 				cfg.Budget.MonthlyUSD = &b
 			}
 		}
+	case settingsFieldAutoRefresh:
+		cfg.TUI.AutoRefresh = val == "true" || val == "1" || val == "yes"
+		a.autoRefresh = cfg.TUI.AutoRefresh
+	case settingsFieldRefreshInterval:
+		var interval int
+		if _, err := fmt.Sscanf(val, "%d", &interval); err == nil && interval >= 10 {
+			cfg.TUI.RefreshIntervalSec = interval
+			a.refreshInterval = time.Duration(interval) * time.Second
+		}
 	}
 
 	a.settings.saveErr = config.Save(cfg)
@@ -184,6 +209,13 @@ func (a App) renderSettingsTab(cw int) string {
 		}
 	}
 
+	// Use live App state for TUI-specific settings (auto-refresh, interval)
+	// to ensure display matches actual behavior after R toggle
+	refreshIntervalSec := int(a.refreshInterval.Seconds())
+	if refreshIntervalSec < 10 {
+		refreshIntervalSec = 30 // match the effective default
+	}
+
 	fields := []field{
 		{"Admin API Key", apiKeyDisplay},
 		{"Session Key", sessionKeyDisplay},
@@ -195,6 +227,8 @@ func (a App) renderSettingsTab(cw int) string {
 			}
 			return "(not set)"
 		}()},
+		{"Auto Refresh", strconv.FormatBool(a.autoRefresh)},
+		{"Refresh Interval", fmt.Sprintf("%ds", refreshIntervalSec)},
 	}
 
 	var formBody strings.Builder