Add SD history UI and pin remap

- Add SD history chart + download listing to web UI
- Use HSPI for SD and fix SD pin mapping
- Swap role/OLED control pins and update role detection
- Update README pin mapping and SD/history docs

README.md

@@ -21,17 +21,23 @@ Variants:
   - SCL: GPIO22
   - RST: **not used** (SSD1306 init uses `-1` reset pin)
   - I2C address: 0x3C
+- microSD (on-board)
+  - CS: GPIO13
+  - MOSI: GPIO15
+  - SCK: GPIO14
+  - MISO: GPIO2
 - I2C RTC (DS3231)
   - SDA: GPIO21
   - SCL: GPIO22
   - I2C address: 0x68
 - Battery ADC: GPIO35 (via on-board divider)
-- **Role select**: GPIO13 (INPUT_PULLDOWN)
-  - LOW = Sender
-  - HIGH = Receiver
-- **OLED control**: GPIO14 (INPUT_PULLDOWN)
+- **Role select**: GPIO14 (INPUT_PULLDOWN, sampled at boot)
+  - HIGH = Sender
+  - LOW/floating = Receiver
+- **OLED control**: GPIO13 (INPUT_PULLDOWN, sender only)
   - HIGH = force OLED on
   - LOW = allow auto-off after timeout
+  - Not used on receiver (OLED always on)
 - Smart meter UART RX: GPIO34 (input-only, always connected)
 
 ### Notes on GPIOs
@@ -257,6 +263,8 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
   - `/sender/<device_id>`: per-sender details
 - Sender IDs on `/` are clickable (open sender page in a new tab).
 - In STA mode, the UI is also available via the board’s IP/hostname on your WiFi network.
+- Main page shows SD card file listing (downloadable).
+- Sender page includes a history chart (power) with configurable range/resolution/mode.
 
 ## MQTT
 - Topic: `smartmeter/<deviceId>/state`
@@ -293,6 +301,8 @@ Key timing settings in `include/config.h`:
  - `SERIAL_DEBUG_MODE` / `SERIAL_DEBUG_DUMP_JSON`
  - `LORA_SEND_BYPASS` (debug only)
  - `ENABLE_SD_LOGGING` / `PIN_SD_CS`
+ - `SD_HISTORY_MAX_DAYS` / `SD_HISTORY_MIN_RES_MIN`
+ - `SD_HISTORY_MAX_BINS` / `SD_HISTORY_TIME_BUDGET_MS`
 
 ## Limits & Known Constraints
 - **Compression**: MeterData uses lightweight RLE (good for JSON but not optimal).
@@ -309,6 +319,9 @@ Optional CSV logging to microSD (FAT32) when `ENABLE_SD_LOGGING = true`.
 - Columns:
   `ts_utc,p_w,p1_w,p2_w,p3_w,e_kwh,bat_v,bat_pct,rssi,snr,err_m,err_d,err_tx,err_last`
 - `err_last` is written as text (`meter`, `decode`, `loratx`) only on the last sample of a batch that reports an error.
+- Files are downloadable from the main UI page.
+- History chart on sender page stream-parses CSVs and bins data in the background.
+- SD uses the on-board microSD SPI pins (CS=13, MOSI=15, SCK=14, MISO=2).
 
 ## Files & Modules
 - `include/config.h`, `src/config.cpp`: pins, radio settings, sender IDs
@@ -331,7 +344,7 @@ Optional CSV logging to microSD (FAT32) when `ENABLE_SD_LOGGING = true`.
 1. Set role jumper on GPIO13:
    - LOW: sender
    - HIGH: receiver
-2. OLED control on GPIO14:
+2. OLED control on GPIO13:
    - HIGH: always on
    - LOW: auto-off after 10 minutes
 3. Build and upload: