SQLBI Whiteboard 0.1.0 (dev 3211)

Add a temporary landing page for whiteboard.sqlbi.com (#11)

A deliberately small page: what the product is, a download button, and a
way to reach every
release. Something to improve on rather than something finished.

**The download cannot be a static link.** The installer file name
carries the version, and
`/releases/latest` returns 404 until a non-prerelease exists — which it
does not yet. The page
therefore resolves the download in the browser from the releases API,
picking the newest
stable release, or the newest pre-release when there is no stable one,
and labelling the
latter plainly as a preview. Without scripting, or if the API is
unreachable, every link
falls back to the releases page, which always works.

The selection logic was checked against the live API response: from
`v0.1.0-dev.3209` it picks
`SQLBI.Whiteboard.0.1.0.x64-dev.msi` at 60 MB, with the per-user
installer and portable ZIP as
secondary links, and shows the preview notice.

**`site/` is now one deployable folder.** The favicons and social card
were generated into
`assets/web`, which would have meant assembling the site from two places
at deploy time. The
generator writes them into `site/` instead, and `assets/` is gone. The
application and
installer assets regenerated byte-identically.

Colours, the icon, and the social card all come from the same brand
source as the
application, so the page and the product match without maintaining a
second palette.