Show tab-aware loading state for Seat Analysis and User Metrics

[Copilot]

The dashboard only showed a loading indicator for metricsReady (default tab path), so switching early to Seat analysis or User metrics could render an empty panel with no feedback. This change makes loading feedback follow the active tab’s own readiness state.

• Loading state logic (centralized)

  • Added shouldShowActiveTabLoading(...) in app/utils/tabUtils.ts.
  • Logic now maps loading visibility to the active tab:
    • seat analysis → !seatsReady
    • user metrics → !userMetricsReady
    • all other tabs → !metricsReady
  • Keeps auth-gated behavior (signInRequired) unchanged.

• Main component wiring

  • Updated app/components/MainComponent.vue to use computed showActiveTabLoading for the shared <v-progress-linear>.
  • Replaced the prior hardcoded !metricsReady && !signInRequired condition.

• Regression coverage

  • Added tests/tabUtils.loading.spec.ts to lock behavior for:
    • seat-analysis loading state
    • user-metrics loading state
    • ready-state suppression
    • sign-in-required suppression

export function shouldShowActiveTabLoading(state: ActiveTabLoadingState): boolean {
  if (state.signInRequired) return false
  if (state.tab === 'seat analysis') return !state.seatsReady
  if (state.tab === 'user metrics') return !state.userMetricsReady
  return !state.metricsReady
}

• Fixes Switching to Seat Analysis / User Metrics tab before its data loads shows a blank screen — no loading feedback #456