Remove unused manifest version field

[dubsector]

Summary

• Neither Windows nor Android's Chromium PWA installers read the manifest `version` field for their app-listing version display — both hardcode 1/1.0 regardless of manifest content (confirmed by building locally and inspecting `dist/manifest.webmanifest`, which had the correct date-based value, and by checking an installed instance on both platforms).
• Removes `getBuildVersion()` and the manifest `version` field along with its `@ts-expect-error` workaround, since it wasn't accomplishing anything.
• The in-app footer's build date/commit stamp (`BUILD_INFO`) is untouched — that one is visible and works.

Test plan

• `npm run build` succeeds, `dist/manifest.webmanifest` no longer has a `version` key
• Verified in preview: app loads, no console errors, footer build info unchanged