Add LILYGO T-Display-C5 board definition

[omegaconjecture]

Proposed Changes

Adds a board definition for the LILYGO T-Display-C5 — an ESP32-C5 board with a 1.9" ST7789 170x320 SPI display and an AXP2602 battery management unit. This lets Bruce build and run on the T-Display-C5 out of the box (pio run -e lilygo-t-display-c5). Includes pins_arduino.h, interface.cpp (custom two-button input handler + backlight/GPIO setup), and the PlatformIO env.

Types of Changes

New Feature (new board support). No changes to shared/core code, so no impact on existing boards.

Verification

Build: pio run -e lilygo-t-display-c5 compiles cleanly.
Flash: uploads over USB-Serial/JTAG; boots to the main menu.
Confirmed on hardware: display, backlight, two-button navigation, and theme colors all render correctly.

Testing

Manually tested on physical hardware (ESP32-C5 rev v1.2):

• Display working over SPI at 20MHz (ST7789 170x320)
• Backlight on/off
• Two-button navigation (IO0 = Next/Prev, BOOT = Select/Back)
• Colors correct with TFT_BGR order
• Clean boot to main menu, no crashes

Linked Issues

None.

User-Facing Change

Add board support for the LILYGO T-Display-C5 (ESP32-C5, 1.9" ST7789 170x320, AXP2602 BMU).

Further Comments

A few board-specific notes and known limitations for reviewers:

• TFT_MISO must be assigned a real GPIO (not -1), or the ESP32-C5 SPI bus fails to initialize and the panel stays blank even though the backlight is on.
• SPI clock is capped at 20MHz; 40MHz produced a blank panel via TFT_eSPI on the C5.
• Backlight is on/off only (digitalWrite). analogWrite/LEDC auto-attach on the C5 left the BL pin undriven, so PWM dimming is disabled for now.
• The on-screen keyboard has no native two-button path yet (Bruce only implements 1- and 3-button keyboards); menu navigation is unaffected.
• External radios (CC1101/NRF24/W5500) share the display SPI bus (one at a time). Probing an absent radio on boot causes minor, harmless display flicker.