Build: felitronics-appkit v0.18.0 — the About window's address at its foot

[AliceLafox]

felitronics-appkit v0.16.0 → v0.18.0. A pin bump; the badge's public surface is unchanged.

• appkit push: opt-in loudness normalization to -18 LUFS (ReplayGain "89 dB") #53 fixes the About window: the product address goes on a line of its own at the window's foot instead of crowding the version row.
• appkit push: accept MP3 for import (bundle minimp3 on all platforms) #52 adds a shared bottom-bar build stamp (VersionStamp) to the family; LooperCat does not adopt it in this PR.

Verified: fresh build directory with every dependency fetched by tag (fetching v0.18.0 in the configure log), app + all 22 ctest suites green on macOS, About window rendered headless.