Add SoftwareApplication (+ SoftwareSourceCode) JSON-LD to the homepage

[IEvangelist]

Summary

Aspire is a developer tool, but the homepage JSON-LD only emits Organization + WebSite. SoftwareApplication / SoftwareSourceCode are high-leverage schema types for developer products (consumed by Google and AI engines) and are currently absent from the homepage.

Evidence / current state

• Live https://aspire.dev/ emits exactly two JSON-LD blocks: Organization and WebSite.
• src/frontend/src/utils/structured-data.ts already has a buildSoftwareApplication() helper (applicationCategory: 'DeveloperApplication', operatingSystem) — but it's only wired into the FAQ, aspireconf, TechArticle, and release-notes schemas via buildStructuredData(), not into buildHomePageSchema() (lines ~81–107).
• SoftwareSourceCode, offers, and codeRepository do not appear anywhere in the codebase.

Where the change goes

src/frontend/src/utils/structured-data.ts:

1. Extend buildHomePageSchema() (lines ~81–107) to add a SoftwareApplication node (and optionally SoftwareSourceCode) into the @graph, wired to the org via @id (e.g. publisher/author/about → ${siteUrl}/#org, which is already defined as organizationId).
2. Enrich buildSoftwareApplication() (lines ~310–320) with:
   • offers → { '@type': 'Offer', price: '0', priceCurrency: 'USD' } plus isAccessibleForFree: true (free / OSS).
   • sameAs and/or codeRepository → the GitHub repos already listed in sameAsLinks (https://github.com/microsoft/aspire, https://github.com/dotnet/aspire).
3. Update tests/unit/structured-data.vitest.test.ts — the home page describe block (lines ~67–99) currently asserts only Organization + WebSite; add assertions for the new SoftwareApplication (and SoftwareSourceCode) node and its @id wiring to #org.

Done when

• The homepage passes Schema.org / Google Rich Results validation with no errors.
• The homepage @graph includes a SoftwareApplication node (and, if added, SoftwareSourceCode) referencing #org, with offers and codeRepository/sameAs populated.

_{Source: SEO audit of https://aspire.dev/ (2026-07-29). Priority: Medium.}