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/config/config.go

@@ -16,6 +16,7 @@ type Config struct {
 	ClaudeAI   ClaudeAIConfig   `toml:"claude_ai"`
 	Budget     BudgetConfig     `toml:"budget"`
 	Appearance AppearanceConfig `toml:"appearance"`
+	TUI        TUIConfig        `toml:"tui"`
 	Pricing    PricingOverrides `toml:"pricing"`
 }
 
@@ -48,6 +49,12 @@ type AppearanceConfig struct {
 	Theme string `toml:"theme"`
 }
 
+// TUIConfig holds TUI-specific settings.
+type TUIConfig struct {
+	AutoRefresh        bool `toml:"auto_refresh"`
+	RefreshIntervalSec int  `toml:"refresh_interval_sec"`
+}
+
 // PricingOverrides allows user-defined pricing for specific models.
 type PricingOverrides struct {
 	Overrides map[string]ModelPricingOverride `toml:"overrides,omitempty"`
@@ -72,6 +79,10 @@ func DefaultConfig() Config {
 		Appearance: AppearanceConfig{
 			Theme: "flexoki-dark",
 		},
+		TUI: TUIConfig{
+			AutoRefresh:        true,
+			RefreshIntervalSec: 30,
+		},
 	}
 }