chore: TUI screenshots, README polish, changelog rewrite (#32)

- Add 6 TUI screenshots to assets/ (monitors, alerts, logs, nodes, detail, theme)
- Rewrite README with hero image, badges, collapsible install sections
- Rewrite changelog to match actual CalVer tag history
- VHS tooling extracted to lerko/uptop-vhs

Reviewed-on: lerko/uptop#32

internal/cluster/cluster_test.go

@@ -53,8 +53,12 @@ func (m *mockStore) GetNode(string) (models.ProbeNode, error) { return models.Pr
 func (m *mockStore) GetAllNodes() ([]models.ProbeNode, error) { return nil, nil }
 func (m *mockStore) UpdateNodeLastSeen(string) error          { return nil }
 func (m *mockStore) DeleteNode(string) error                  { return nil }
-func (m *mockStore) SaveLog(string) error                     { return nil }
-func (m *mockStore) LoadLogs(int) ([]string, error)           { return nil, nil }
+func (m *mockStore) LoadAlertHealth() (map[int]models.AlertHealthRecord, error) {
+	return nil, nil
+}
+func (m *mockStore) SaveAlertHealth(models.AlertHealthRecord) error { return nil }
+func (m *mockStore) SaveLog(string) error                           { return nil }
+func (m *mockStore) LoadLogs(int) ([]string, error)                 { return nil, nil }
 func (m *mockStore) GetActiveMaintenanceWindows() ([]models.MaintenanceWindow, error) {
 	return nil, nil
 }