Web-UI: show SD card storage usage

[joker-mik]

Summary

Adds storage usage information for the SD card to the Files section of the web UI.

The display shows:

• used storage
• total storage
• free storage
• usage percentage
• progress bar

Storage values are displayed using decimal units (MB / GB).

Implementation

• add helpers for SD card mounted state, total filesystem size and used size
• expose storage information through /info?section=sdcard
• display storage usage above the file browser
• refresh the information when opening the Files/RFID tab and after file operations
• add German, English and French translations
• handle unavailable SD cards and unavailable filesystem statistics

Testing

Tested on real ESPuino hardware with an SD card.

• storage capacity and usage are displayed correctly
• file browser continues to work normally
• storage information updates after file operations
• tested with desktop browser and Android/Chrome