feat(pi): mount USB drives in Files without auto data-disk migration

[LuckyCoders]

Summary

Raspberry Pi currently uses a boot-time script (umbrel-external-storage) that treats any single USB disk as a Umbrel data disk (wipe + mkfs.ext4 -L umbrel). The Files app external storage feature is disabled on Pi in both backend and UI.

This PR separates the two flows:

• Default (no opt-in): USB drives are left alone at boot and mounted by umbreld under Files → External, like on Umbrel Home / x86.
• Opt-in only: The legacy data-disk migration script runs only if umbrel-allow-external-format exists on the SD card boot partition (same flag as the format-safety PR).

Fixes the common case where users plug in a drive with existing data (NTFS/exFAT/ext4) expecting removable storage, not a wiped Umbrel data volume.

Depends on: #2148 (fix/pi-refuse-auto-format-external-usb)

Problem

On Pi, connecting a USB drive could trigger:

1. wipefs + GPT + mkfs.ext4 -L umbrel (if not ext4 / no .umbrel)
2. UI message: “External storage not supported”
3. No way to use the drive in Files without erasing it

Changes

Boot script (umbrel-external-storage)

• Exit immediately (success) when umbrel-allow-external-format is not present
• Log that USB is handled by umbreld under Files → External
• Data-disk migration (format + bind mount /home/umbrel) only runs with explicit opt-in

umbreld external storage

• Enable external storage module on Raspberry Pi (supported() → true)
• Run UAS blacklist check on Pi start (moved from boot script path when data-disk script is skipped)
• Exclude disks already mounted at /mnt/data (Umbrel data volume) from /External
• New API: files.isExternalStorageSupported

UI

• Use files.isExternalStorageSupported instead of hardcoded productName !== 'Raspberry Pi'
• External storage sidebar / eject works on Pi when OS is updated

OS image

• Install ntfs-3g on arm64 (was amd64-only)