feat: add board configuration system, refactor Config.h

Board selection driven by -DTARGET_E32R35T or -DTARGET_WAVESHARE_S3_43.
Defaults to E32R35T for backward compatibility.

All hardware pin definitions moved from Config.h to board headers.
SCREEN_WIDTH/SCREEN_HEIGHT now come from board headers.
Config.h is now purely application-level configuration.

New files:
  BoardConfig.h              — board selector
  boards/board_e32r35t.h     — E32R35T pins, display, touch config
  boards/board_waveshare_s3.h — Waveshare S3 4.3" pins, display, touch config

Modified:
  Config.h — removed hardware pins section, added #include BoardConfig.h,
             added HOLD_DURATION_MS to timing section

sketches/doorbell-touch-esp32-32e/boards/board_e32r35t.h

@@ -0,0 +1,50 @@
+#pragma once
+// ═══════════════════════════════════════════════════════════════════
+// Board: E32R35T — ESP32-WROOM-32E + 3.5" ST7796S SPI + XPT2046
+// ═══════════════════════════════════════════════════════════════════
+
+#define BOARD_NAME            "E32R35T"
+
+// ── Display ─────────────────────────────────────────────────────
+#define SCREEN_WIDTH          480
+#define SCREEN_HEIGHT         320
+#define DISPLAY_ROTATION      1
+
+// ── Driver selection ────────────────────────────────────────────
+#define USE_TFT_ESPI          1
+#define USE_ARDUINO_GFX       0
+#define USE_TOUCH_XPT2046     1
+#define USE_TOUCH_GT911       0
+
+// ── Hardware capabilities ───────────────────────────────────────
+#define HAS_PSRAM             0
+
+// ── LCD (HSPI) ──────────────────────────────────────────────────
+#define PIN_LCD_CS    15
+#define PIN_LCD_DC     2
+#define PIN_LCD_MOSI  13
+#define PIN_LCD_SCLK  14
+#define PIN_LCD_MISO  12
+#define PIN_LCD_BL    27
+
+// ── Touch (XPT2046, shares HSPI) ───────────────────────────────
+#define PIN_TOUCH_CS  33
+#define PIN_TOUCH_IRQ 36
+
+// ── SD Card (VSPI — for future use) ────────────────────────────
+#define PIN_SD_CS     5
+#define PIN_SD_MOSI   23
+#define PIN_SD_SCLK   18
+#define PIN_SD_MISO   19
+
+// ── RGB LED (active low) ───────────────────────────────────────
+#define PIN_LED_RED   22
+#define PIN_LED_GREEN 16
+#define PIN_LED_BLUE  17
+
+// ── Audio ───────────────────────────────────────────────────────
+#define PIN_AUDIO_EN   4
+#define PIN_AUDIO_DAC 26
+
+// ── Battery ADC ─────────────────────────────────────────────────
+#define PIN_BAT_ADC   34