feat: implement true sleep timer with hardware mute and status icon

[bostoneboy]

Overview
This PR introduces a dedicated Sleep Timer feature, allowing users to set a countdown (1-120 minutes) after which the device will automatically stop playback and enter
sleep mode. This is distinct from the existing inactivity-based display sleep.

Key Features

• True Sleep Logic: When the timer expires, it performs a "long-press" action—turning off the display and triggering a hardware mute (disabling the audio amplifier) to
  ensure zero power consumption and noise.
• Dynamic Stepping & Wrap-around:
  • 1-minute increments for the first 20 minutes.
  • 5-minute increments from 20 to 120 minutes.
  • Seamless wrap-around logic (e.g., 120 min -> Off -> 1 min) for easier navigation.
• Intelligent Volume Restoration: Automatically restores the previous volume level upon waking up (soft power-on) if the device was muted by the Sleep Timer, while
  preserving manual mute states.
• Visual Indicator: Added a "Zz" icon in the status bar (top bar) that appears only when the Sleep Timer is active.
• Persistence: The Sleep Timer setting is saved to NVS and persists across reboots.

Changes

• ats-mini.ino: Added timer countdown logic and state management in the main loop.
• Menu.cpp / Menu.h: Added "Sleep Timer" to the Settings menu with custom encoder handling and UI rendering.
• Utils.cpp: Enhanced sleepOn() to handle volume restoration using a new sleepTimerMuted flag.
• Draw.cpp / Draw.h: Implemented and integrated the "Zz" status icon.
• Storage.cpp: Added persistence for the new timer setting.