fix(installer): harden the disk-space pre-flight gate

`df /` wraps to two lines when the root device name is long -- a device-mapper or live-ISO root -- so `awk 'NR==2 {print $4}'` read the wrapped device line, found an empty Available field, and aborted a valid install with "Insufficient disk space." `df -P` forces POSIX single-line output.

The gate now compares available KB against the minimum in KB rather than truncating to GB first, which rejected a disk sitting just under a whole-GB boundary. Non-numeric df output falls back to zero so a malformed read aborts loudly instead of crashing the arithmetic test.

Extracted the logic into check_disk_space so it can be exercised directly.

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