Fix: Estimated AQI now matches Current AQI when Scenario Simulator sliders are at 0%

[AimaMuzammil]

Summary

Fixes #113 — The Scenario Simulator was showing a reduced "Estimated AQI"
even when all intervention sliders were at 0% and no policy scenario was
selected.

Root Cause

The estimatedAqi value was always recalculated using the local
estimateAQI() function, even when no reduction was applied. Since this
function uses a different formula than the one used by the Open-Meteo API
(which provides current.us_aqi), the two values could differ slightly
even with identical pollutant inputs.

Fix

Added a hasReduction check: if all sliders are at 0% (no reduction
selected), estimatedAqi now uses current.us_aqi directly instead of
recalculating it. The same fix was applied to the simulateCity()
function used for the city comparison cards, to prevent the same issue
there.

Testing Done

• ✅ With all sliders at 0%: Current AQI and Estimated AQI now match exactly
• ✅ Tested all 5 presets (Traffic Control, Industrial Limits, Green Cover,
  Clean Fuel Switch, Combined Policy) — Estimated AQI still correctly
  decreases based on the selected reduction
• ✅ Tested city comparison cards (Delhi, Mumbai, Bengaluru, Kolkata,
  Chennai, Hyderabad) — all showing correct before/after values

Before (bug):
Sliders at 0%, but Estimated AQI shows a fake -29% reduction (77 → 55)

After (fixed):
Sliders at 0%, Current AQI and Estimated AQI now match exactly (77 → 77)

---

I'm an ECSoC'26 contributor in the UI Components track.

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[AimaMuzammil]

Hi @Aditya8369! This PR fixes Issue #113. Could you please review it
whenever you get a chance, and add the ECSoC26 label if everything looks
good? Thank you!

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