Improve script robustness, fix edge cases, and pass shellcheck

[zbcoding]

Summary

This PR builds on the great work in #10 by @rogerb831 (and earlier contributions from @Cristy94 ), which added pigz support, interactive prompts, multi-distro detection, and comprehensive documentation. Those are really nice additions to the migration script.

This PR layers on some additional hardening and fixes a few edge cases discovered
while testing:

• Add set -euo pipefail to catch errors early instead of silently continuing
• Replace string-manipulation conditionals (${answer#[Yy]}) with clearer case
  statements for y/n prompts
• Use $(id -u) instead of $EUID for better POSIX compatibility
  • Change StrictHostKeyChecking no to StrictHostKeyChecking accept-new (avoids
    silently accepting changed host keys — a potential security concern)
• Add a disk space check before creating the backup
  • Check if ~/.ssh/authorized_keys exists locally before including it in the tar
    archive
• Add mkdir -p ~/.ssh on the remote server before writing keys
• Add </dev/null redirects for remote package-install commands to prevent them from consuming the backup stream piped over stdin
• Track whether Docker was stopped and prompt to restart it on the source server
  after migration
• Add SSH keepalive options to prevent disconnects during large transfers
• Skip redundant backup creation if the archive already exists (e.g., from a previous interrupted run) - Clean up authorized_keys_backup temp file and set proper permissions on ~/.ssh
• Add a final success message
• Pass shellcheck cleanly (with targeted disable comments where needed)

Test plan

• Run migration on a test Coolify instance (Debian-based)
• Verify pigz install prompt and fallback to gzip both work
• Verify backup skips creation when archive already exists
• Verify Docker restart prompt appears on source server
• Confirm authorized_keys are correctly merged on destination
• Test with low disk space to confirm warning triggers

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