Snap: auto-promote release builds to stable after candidate QA

[jatinkrmalik]

Why

#519 landed Snap packaging and CI publish on v* tags to edge + candidate only. stable is still a manual snapcraft release … stable after QA.

Once we trust candidate (install, tray, mic, model download, type into a real app on Ubuntu), we should automate stable too.

Options

1. Simple: change publish-snap in .github/workflows/release.yml to release: edge,candidate,stable.
2. Safer: keep auto candidate; add workflow_dispatch (or Environment approval) promote job for stable.

Recipe already has grade: stable, so the Store will accept stable channel releases.

Done when

• Documented decision (1 or 2)
• Wired in CI
• First v* after that lands on latest/stable without a hand snapcraft release

Refs

• Parent: 📦 Publish Vocalinux to Ubuntu Snap Store #48
• PR: feat(snap): Snap packaging recipe and Snap Store publish strategy #519
• Store: https://snapcraft.io/vocalinux
• Current channels: edge 0.16.2; candidate/stable empty until next tag / promote