Add DFU screen support for I2C mono OLEDs

[rcarteraz]

Boards with an I2C OLED currently show nothing while sitting in DFU mode. This adds an SSD1306/SH1106 display path alongside the existing SPI TFT one, so those boards render the same UF2 drag-and-drop and BLE OTA screens the Heltec boards already get.

Boards

Enabled	Notes
RAK4631	Validated on hardware
RAK3401	Pin-identical WisBlock sibling, untested
Wio Tracker L1	Variant declares HAS_SCREEN 1 / USE_SSD1306 1, untested
ProMicro nRF52840	Pins per the default Meshtastic variant, untested
XIAO nRF52840 / Sense	Kit-default I2C pins, untested

Boards with no display (SenseCAP Solar P1, T1000-E, ThinkNode M3/M6, WisMesh Tag, MX25LE01) build byte-identical — the whole feature is behind a board.h declaration. E-ink boards (T-Echo, ThinkNode M1) are unaffected and still need a separate e-paper driver.

Notes

Panel presence is probed rather than assumed, since an OLED is often a plug-in module or user-wired. If nothing ACKs, the screen is skipped and the board behaves exactly as before; every I2C wait is bounded so a missing or unterminated bus can't stall the bootloader.

Layout defaults for 1bpp panels live in screen.c, so a board.h only declares its bus, geometry and title. The existing colour layout survives the mono conversion unchanged because only the foreground palette entries light a pixel.

Cost is ~1.8KB of flash on boards that opt in. The tightest board here ends at 94.3%.

DISPLAY_COL_OFFSET is panel-specific and can't be detected — the RAK4631's panel turned out to be 132-column, needing an offset of 2. The untested boards default to 0 with blanking spanning 132 columns, so a mismatch shows as a 2px shift rather than stray pixels. Whoever tests each board flips that if the image sits left.

Also included

An out-of-bounds write in printicon(): on the 160px-wide T096 and T1, DRAGX 4 plus the default pendriveLogo_X 129 put the third icon at x=133..164, writing 386 bytes past the end of frame_buf. Fixed by moving it to 124 and bounds-checking the blit. Predates this work and is independent of it — happy to split it out if preferred.

Testing

All 17 boards build clean under -Werror. RAK4631 flashed and confirmed showing both screens; layout verified by compiling screen.c natively and dumping the pages the driver emits.

[coderabbitai]

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