Release readiness for v0.1.0: icon, About, help, user guide, winget

[PlanetLinux98]

Everything that has to be true before v0.1.0 can be tagged, beyond the features.

Checklist

• App icon: a simple placeholder is in place now; a proper design (a few concepts, SVG master, generated .ico with 16 to 256 px sizes) before release. The form uses the exe's embedded icon so nothing ships loose.
• About dialog: version, short description, licences and attribution (Open-Meteo CC BY 4.0, MeteoAlarm CC BY 4.0, NWS, Environment Canada), link to the repository.
• Help (F1): opens the user guide. For 0.1.0 the guide is USER_GUIDE.md in the repository, opened in the browser; shipping it inside the exe is a later option.
• USER_GUIDE.md: setup, every feature and shortcut, alerts coverage by region, settings, offline behaviour, troubleshooting.
• README updated from "pre-release" to the download and winget instructions.
• CHANGELOG.md: [Unreleased] moved under [0.1.0].
• Tag v0.1.0; CI drafts the Release with the exe and SHA256SUMS; publish.
• First winget submission by hand with wingetcreate new (portable installer type, command alias weatherspell, identifier PlanetLinux98.Weatherspell); winget.yml handles later versions automatically.
• NVDA pass over every window and dialog, plus a run through Narrator.