fix(packaging): re-running setup no longer re-prompts for firewall access

Every run of customInstall showed the "Allow MeshBay through Windows
Firewall?" question and, on Yes, a fresh UAC prompt -- an upgrade or repair
install would ask again even with all four rules already in place.

customInstall now checks first: firewall.ps1 check, unelevated
(Get-NetFirewallRule needs no admin, only New/Remove do), exits 0 if every
rule already exists. Only a nonzero result reaches the MessageBox and the
elevated add. A second run of setup on an already-configured machine now
asks nothing.

Verified unelevated: check exits 1 and logs which rules are missing on a
machine with none of them (the fresh-install case); electron-builder
compiles the nsExec::Exec / Pop $0 / ${If} wiring. Node suite 837 pass /
25 skip.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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