refactor(tui): improve config resilience and scroll navigation

app.go changes:
- Add loadConfigOrDefault() helper that returns sensible defaults when
  config loading fails, ensuring TUI can always start even with
  corrupted config files
- Extract scroll navigation constants (scrollOverhead, minHalfPageScroll,
  minContentHeight) for clarity and consistency
- Apply accent border styling to loading card for visual polish
- Replace inline config.Load() calls with loadConfigOrDefault()

setup.go changes:
- Use loadConfigOrDefault() for consistent error handling during
  setup wizard initialization and config persistence

The loadConfigOrDefault pattern improves user experience by gracefully
degrading rather than failing hard when config issues occur. Users can
still access the TUI and reconfigure via the Settings tab.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/tui/setup.go

@@ -19,7 +19,7 @@ type setupValues struct {
 
 // newSetupForm builds the huh form for first-run configuration.
 func newSetupForm(numSessions int, claudeDir string, vals *setupValues) *huh.Form {
-	cfg, _ := config.Load()
+	cfg := loadConfigOrDefault()
 
 	// Pre-populate defaults
 	vals.days = cfg.General.DefaultDays
@@ -98,7 +98,7 @@ func newSetupForm(numSessions int, claudeDir string, vals *setupValues) *huh.For
 
 // saveSetupConfig persists the setup wizard values to the config file.
 func (a *App) saveSetupConfig() error {
-	cfg, _ := config.Load()
+	cfg := loadConfigOrDefault()
 
 	if a.setupVals.sessionKey != "" {
 		cfg.ClaudeAI.SessionKey = a.setupVals.sessionKey