feat: FTP server with Wi-Fi, NTP, LVGL UI — initial working build

- Rewrite main.cpp: SD_MMC mount, Wi-Fi from JSON, NTP Vienna, FTP server
- Add ui.h/ui.cpp: LVGL status screen (IP, FTP status, SD bar, time)
- Add build_framework_libs.py: dynamic ESP32 Core 3.x lib resolution
- Remove unused sensor libs (IMU, magnetometer, fuel gauge, charger)
- Add SimpleFTPServer + ArduinoJson dependencies
- Build succeeds: RAM 19.2%, Flash 4.2%

extra_scripts/build_framework_libs.py

@@ -0,0 +1,37 @@
+"""
+Pre-build script for Kode Dot FTP Explorer.
+
+Resolves ESP32 Arduino Core 3.x framework library paths dynamically and
+compiles framework libraries that PlatformIO's LDF doesn't auto-discover
+(Network, FFat).  Also adds include paths for all framework libraries
+needed by SimpleFTPServer.
+"""
+Import("env")
+import os
+
+framework_dir = env.PioPlatform().get_package_dir("framework-arduinoespressif32")
+libs_dir = os.path.join(framework_dir, "libraries")
+
+# Framework libraries whose include paths are needed by SimpleFTPServer
+# and our code (some may already be discovered by LDF — adding extras is harmless)
+INCLUDE_LIBS = [
+    "Network", "WiFi", "FS", "SD_MMC", "SD", "SPI",
+    "FFat", "SPIFFS", "LittleFS",
+]
+
+for lib_name in INCLUDE_LIBS:
+    inc_dir = os.path.join(libs_dir, lib_name, "src")
+    if os.path.isdir(inc_dir):
+        env.Append(CPPPATH=[inc_dir])
+
+# These libraries are NOT auto-discovered by PlatformIO's LDF
+# and must be explicitly compiled
+COMPILE_LIBS = ["Network", "FFat"]
+
+for lib_name in COMPILE_LIBS:
+    src_dir = os.path.join(libs_dir, lib_name, "src")
+    if os.path.isdir(src_dir):
+        env.BuildSources(
+            os.path.join("$BUILD_DIR", "Framework" + lib_name),
+            src_dir,
+        )