fix(build.func): expand glob in SSH key "Scan Folder/Glob" so it can find keys

[TowyTowy]

✍️ Description

The advanced-install SSH key menu offers "folder — Scan another folder (path or glob)", but the input is split with:

shopt -s nullglob
read -r -a _scan_files <<<"$glob_path"
shopt -u nullglob

read performs no pathname expansion (nullglob has no effect on it), so _scan_files holds the literal pattern and ssh_build_choices_from_files() runs [[ -f ... ]] against it. Both advertised inputs therefore always fail: a glob like /root/.ssh/*.pub is tested as a literal filename, and a plain folder fails -f because it is a directory. The option unconditionally ends in "No keys found" and has never been able to import a key (behavior introduced in #9540).

This PR expands the pattern with compgen -G — the same glob-existence idiom as #15841 — and treats a directory input as dir/*, so both input forms shown in the input box work.

Standalone repro (folder containing one.pub, two backup.pub, known_hosts):

input: /tmp/keys/*.pub   -> old code finds 0 key files, fixed code finds 2
input: /tmp/keys         -> old code finds 0 key files, fixed code finds 2

shellcheck findings on misc/build.func: 34 before, 34 after (no new findings); bash -n clean under bash 5.2. Testing scope: verified via the standalone repro (buggy and fixed blocks lifted verbatim) and shellcheck; not run through the full whiptail flow on a live Proxmox host.

Disclosure: this fix was prepared with AI assistance (Claude); the bug, repro, and diff were verified by running the shown commands.

🔗 Related Issue

Fixes # (none filed — found via shellcheck-guided audit; behavior introduced in #9540)

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected (scope as stated above).
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to script metadata (PocketBase/website data).
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.