Add Reference Frequency Fine Tuning

[pbalduino]

Description

Allow fine adjustments to A4 reference frequency in ±0.5 Hz increments

Priority

🟢 Low

Category

Feature

Details

• Extend settings screen to allow A4 frequency adjustment
• Support range from 430-450 Hz with 0.5 Hz increments
• Update frequency calculations throughout the app
• Save preference to local storage

Files Affected

• lib/screens/settings_screen.dart
• lib/screens/tuner_screen.dart

Acceptance Criteria

• Settings screen shows A4 frequency control
• Frequency can be adjusted in 0.5 Hz increments
• All frequency calculations use the set reference
• Setting persists between app restarts

Task ID: CONFIG-001