feat(web): Navigator download page with per-platform installer links

[JamesKane]

Adds /download — Windows / macOS / Linux installers for the Navigator edge app — plus /download/{windows,macos,linux}, permanent redirects that always land on the current build.

Why it isn't just three static links

The obvious approach (…/releases/latest/download/<asset>) fails twice over for this repo:

1. GitHub's "latest" excludes pre-releases, and every Navigator installer release is an alpha pre-release. Today /releases/latest resolves to assets-chm13v2.0 — the Ancestry/IBD + STR reference-data release — not the app.
2. Tauri stamps the app version into every filename (navigator_0.1.0_x64-setup.exe), so there is no stable asset name to hard-code.

So du-external::github lists releases newest-first and takes the first one that actually carries installer assets — drafts skipped, pre-releases kept — then classifies each asset by platform / arch / format and marks a primary build per platform (x64 .exe, universal .dmg, x86_64 AppImage). Updater artifacts (.sig, .app.tar.gz) and data assets are not downloads and are filtered out.

Behaviour

• Caching: resolution is cached process-wide, 30 min on success and 5 min after a failure — roughly 2 GitHub calls/hour against an unauthenticated limit of 60.
• Degradation: if GitHub is unreachable the page shows a warning and falls back to the releases listing; the redirects fall back the same way. Never a 500.
• Platform detection: the visitor's OS is guessed from the User-Agent and their card is shown first and highlighted. Phones get no false "your system" match. Every platform stays visible either way.
• Secondary builds: arm64 AppImage and both .debs are listed under the primary button, with sizes; SHA256SUMS is linked for verification.

Also links the page from the Tools nav, the home "participate" section and the FAQ answer that already mentions the Navigator, and adds /download to the sitemap. That FAQ answer claimed the Navigator is "built on the JVM (Java/Scala)" — corrected to Rust, since it sits directly above the new download button.

Config: DU_NAVIGATOR_REPO overrides the source repo (default JamesKane/decodingus-navigator); GITHUB_TOKEN, if set, raises the API rate limit. Neither is required.

Verification

Run against the live repo (cargo run -p du-web, real GitHub API):

/download          → v0.1.0-alpha.15, all 6 installers + SHA256SUMS, Alpha badge
/download/windows  → 307 → …/v0.1.0-alpha.15/navigator_0.1.0_x64-setup.exe → 200 attachment
/download/macos    → 307 → …/DUNavigator_0.1.0_universal.dmg
/download/linux    → 307 → …/navigator_0.1.0_x86_64.AppImage
/download/bogus    → 307 → …/releases   (unknown slug → listing, not a 404)

macOS UA highlighted the macOS card; Windows UA reordered Windows first.

Tests: cargo test -p du-external (26 pass — 7 new, covering the pick-the-installer-release rule against a fixture of the real response shape, asset classification, draft/updater filtering, and the empty case) and cargo test -p du-web --bins (71 pass — 6 new for the view model, template, UA detection and size formatting, plus the en/es/fr locale-parity test over the 18 new keys).

No migration, no schema or API change.

🤖 Generated with Claude Code