Optimize display rendering pipeline and introduce PWM_DEPTH config

[JanPetterMG]

This PR introduces a configurable PWM_DEPTH setting and significantly optimizes the display rendering pipeline by shifting to a bitplane-based approach. These changes reduce the computational overhead during high-frequency hardware interrupts, resulting in smoother rendering and more efficient CPU usage.

Key Changes

• Configurable PWM Depth: Added PWM_DEPTH support to config/secrets.h and config/services.h, allowing developers to manually dictate the PWM bit depth rather than relying strictly on automatic calculations.
• Bitplane Rendering Pipeline: Refactored DisplayService::onTimer() and flush() to pre-calculate and process display frames into bitplanes (planes[plane]). This allows the high-frequency timer interrupt to simply push pre-formatted bytes via SPI.
• Hardware Timer & SPI Adjustments: Updated the SPI speed and timer alarm configurations in DisplayService.cpp to dynamically align with the newly established PWM_DEPTH and frame rate variables.
• Code Cleanup: Standardized array iteration logic (e.g., swapping pixel for idx) and minor refactoring across the core display and extension services.

Impact

• Performance: Extracting the heavy bitwise brightness thresholding logic from onTimer() severely reduces the time spent inside the interrupt service routine (ISR).
• Customization: Users can now tailor the balance between visual depth and performance based on their specific hardware capabilities by adjusting PWM_DEPTH.