Fix Microsoft Store update discovery

[danielss-dev]

Summary

• query Microsoft Store package updates for MSIX installs on launch and from Settings
• notify users when a Store update is available and open Strand's Store product page
• preserve the signed GitHub Releases updater for direct installations
• update Store policy checks, product documentation, and regression coverage

Test plan

• cargo check -p strand-core -p strand-tauri
• ui/node_modules/.bin/tsc.CMD --noEmit
• ui/node_modules/.bin/vitest.CMD run src/stores/updates.test.ts src/lib/distribution.test.ts src/lib/i18n.test.ts
• node scripts/check-msix.mjs
• isolated MSIX browser preview of Settings → Updates

[danielss-dev]

Superseded by #88, which ported DAN-35 onto current main, fixed the Linux Clippy failure, resolved the DAN-38 ROADMAP conflict, passed all required checks, and merged as 4fff901.