feat(tui): add SLA reporting view

Full-screen SLA report accessible via [s] from detail panel.
Computes uptime%, downtime, outage count, longest outage, MTTR,
and MTBF from state_changes table. Includes daily breakdown with
bar chart, switchable time periods (24h/7d/30d/90d), and
scrollable viewport. LATE/STALE treated as UP for SLA purposes.

internal/server/server_test.go

@@ -82,7 +82,10 @@ func (m *mockStore) GetPreference(string) (string, error)                   { re
 func (m *mockStore) SetPreference(string, string) error                     { return nil }
 func (m *mockStore) SaveStateChange(int, string, string, string) error      { return nil }
 func (m *mockStore) GetStateChanges(int, int) ([]models.StateChange, error) { return nil, nil }
-func (m *mockStore) Close() error                                           { return nil }
+func (m *mockStore) GetStateChangesSince(int, time.Time) ([]models.StateChange, error) {
+	return nil, nil
+}
+func (m *mockStore) Close() error { return nil }
 
 func (m *mockStore) ExportData() (models.Backup, error) {
 	return models.Backup{