Refactor to unified firmware architecture with modular configuration system

BREAKING CHANGE: Replace separate anchor/tag firmware with single configurable codebase

## Major Changes
- **Unified Firmware**: Single main.cpp replaces main_anchor.cpp and main_tag.cpp
- **Modular Config**: Organized configuration system in src/config/ directory
- **Dynamic Behavior**: Device behavior determined by build flags at compile time
- **Simplified Builds**: Reduced from 8+ environments to 3 core environments

## New Architecture
- src/main.cpp - Single firmware with config-driven behavior
- src/config/device_config.h - Device role detection and unified behavior
- src/config/anchor_config.h - Anchor-specific settings (positioning, communication)
- src/config/tag_config.h - Tag-specific settings (USB streaming, coordinate collection)
- src/config/positioning_config.h - Distributed positioning algorithm framework

## Build System Improvements
- Simplified platformio.ini (anchor/tag/debug environments)
- Support for custom device ID and network via PLATFORMIO_BUILD_FLAGS
- Same firmware for all devices, behavior configured at compile time
- Easy device configuration: set PLATFORMIO_BUILD_FLAGS=-DUWB_INDEX=5 -DNETWORK_ID=2000

## Technical Benefits
- Reduced code duplication from ~350 lines to unified ~200 lines
- Consistent behavior across device types
- Easier maintenance with single codebase
- Framework ready for distributed positioning algorithm implementation
- Modular configuration system for future algorithm expansion

## Positioning Framework Ready
- Anchor discovery protocol framework established
- Inter-anchor communication configuration ready
- Position calculation algorithm structure in place
- Coordinate storage and validation system configured
- Tag coordinate collection and USB streaming framework prepared

## Testing
- ✅ Anchor build successful (325KB flash usage)
- ✅ Tag build successful (325KB flash usage)
- ✅ Custom device ID/network configuration working
- ✅ UWB initialization sequence fixed for active networks

src/config/config.h

@@ -0,0 +1,56 @@
+#ifndef CONFIG_H
+#define CONFIG_H
+
+// Hardware Pin Definitions for ESP32-S3
+#define RESET_PIN 16
+#define IO_RXD2 18
+#define IO_TXD2 17
+#define I2C_SDA 39
+#define I2C_SCL 38
+
+// UWB Configuration
+#ifndef UWB_INDEX
+#define UWB_INDEX 0
+#endif
+
+#ifndef NETWORK_ID
+#define NETWORK_ID 1234
+#endif
+
+// System Configuration
+#define MAX_DEVICES 10
+#define MAX_ANCHORS 8
+#define UWB_TAG_COUNT 64
+
+// Timing Configuration
+#define DISPLAY_UPDATE_INTERVAL 500
+#define DEVICE_TIMEOUT 5000
+#define SLEEP_DURATION 5000
+
+// Display Configuration
+#define SCREEN_WIDTH 128
+#define SCREEN_HEIGHT 64
+#define OLED_RESET -1
+#define SCREEN_ADDRESS 0x3C
+
+// Serial Configuration
+#define SERIAL_BAUD 115200
+
+// Debug Configuration
+#ifdef DEBUG_ENABLED
+#define DEBUG_PRINT(x) Serial.print(x)
+#define DEBUG_PRINTLN(x) Serial.println(x)
+#define DEBUG_PRINTF(x, ...) Serial.printf(x, __VA_ARGS__)
+#else
+#define DEBUG_PRINT(x)
+#define DEBUG_PRINTLN(x)
+#define DEBUG_PRINTF(x, ...)
+#endif
+
+// WiFi Configuration (optional)
+#ifdef OTA_ENABLED
+#define WIFI_SSID "YOUR_WIFI_SSID"
+#define WIFI_PASSWORD "YOUR_WIFI_PASSWORD"
+#endif
+
+#endif // CONFIG_H