feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/esp-hal/ld/esp32/rom/esp32.rom.spiflash_legacy.ld

@@ -0,0 +1,29 @@
+/*
+ * SPDX-FileCopyrightText: 2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+/*
+	Address table for SPI driver functions in ESP32 ROM.
+	These functions are only linked from ROM when SPI_FLASH_ROM_DRIVER_PATCH is not set in configuration.
+*/
+
+PROVIDE ( esp_rom_spiflash_write_encrypted = 0x40062e78 );
+PROVIDE ( esp_rom_spiflash_erase_area = 0x400631ac );
+PROVIDE ( esp_rom_spiflash_erase_block = 0x40062c4c );
+PROVIDE ( esp_rom_spiflash_erase_chip = 0x40062c14 );
+PROVIDE ( esp_rom_spiflash_erase_sector = 0x40062ccc );
+PROVIDE ( esp_rom_spiflash_attach = 0x40062a6c );
+PROVIDE ( esp_rom_spiflash_lock = 0x400628f0 );
+PROVIDE ( esp_rom_spiflash_read = 0x40062ed8 );
+PROVIDE ( esp_rom_spiflash_config_readmode = 0x40062b64 ); /* SPIMasterReadModeCnfig */
+PROVIDE ( esp_rom_spiflash_read_status = 0x4006226c );
+PROVIDE ( esp_rom_spiflash_read_statushigh = 0x40062448 );
+PROVIDE ( esp_rom_spiflash_write = 0x40062d50 );
+PROVIDE ( esp_rom_spiflash_enable_write = 0x40062320 );
+PROVIDE ( esp_rom_spiflash_write_status = 0x400622f0 );
+
+/* always using patched versions of these functions
+PROVIDE ( esp_rom_spiflash_wait_idle = 0x400622c0 );
+PROVIDE ( esp_rom_spiflash_unlock = 0x400????? );
+*/