Auto-mount ESP during preflight and UKI build flows

[GlitchSlayed]

Motivation

• Avoid hard-failing when the EFI System Partition (ESP) is present but not mounted so the setup can proceed or self-fix common user setups.
• Ensure the generated /usr/local/sbin/uki-build.sh can also recover when run later (e.g., from kernel-install hooks) if the ESP is not mounted.
• Make documentation reflect the improved behavior so users know the script will attempt to mount the ESP automatically.

Description

• Added find_esp_device (detect ESP partition via lsblk GUID/FSTYPE) and ensure_esp_mounted helpers to uki-setup.sh to try mounting /boot/efi or /efi using fstab first and falling back to mounting the detected ESP device with -t vfat.
• Replaced the old preflight hard-fail with ensure_esp_mounted || die ... so the script attempts automatic mounting before aborting.
• Injected the same find_esp_device / ensure_esp_mounted logic into the generated uki-build.sh template and enforced it before building a UKI so rebuilds also auto-mount the ESP when possible.
• Updated README.md Requirements and Troubleshooting to document the auto-mount behavior and the fallback for manual mounting.

Testing

• Ran shell syntax checks with bash -n uki-setup.sh tests/test_uki_setup.sh which completed successfully.
• Executed the project validation script bash tests/test_uki_setup.sh which passed and confirmed generated templates are populated as expected.
• Attempted shellcheck uki-setup.sh but shellcheck was not available in the environment (not a failure of the change).

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Codex Task