Login screen: rework the background treatment (follow-up to #16)

#16 bundled the login logo and camera-trap background through auth-ui, full-bleed behind the form. In the PR thread @Chris-Schnaufer and I leaned toward **option D** from the mockups: https://claude.ai/code/artifact/d969ee98-e58e-497b-9e2e-3d97f4278dd4
*(screenshot of option D)*

Still open (from @smalusa's comment): if we want SPARCd to appeal to projects with different objectives, should the login have project-specific imagery at all? A jaguar assumes our use case, a new styling could go neutral instead. Deciding that here; option D is the treatment if we keep an image.

Work once decided:

- Implement in `packages/auth-ui/[Connection.tsx](https://github.com/Users/juliangonzalez/dev/sparcd-rewrite/packages/auth-ui/Connection.tsx)` (shared by uploader + tagger)
- Small-screen fallback so mobile login doesn't regress (#6)

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User story: none yet. Candidate: gap-report group 1 (connection & session).

Resolve the open question about project-specific imagery by reviewing the discussion and option D mockup first. Then inspect packages/auth-ui/Connection.tsx, which is shared by the uploader and tagger, and review issue #6 for the mobile fallback requirement. Done means the chosen background treatment is implemented without regressing small-screen login.

Written by the indexing model from the issue text.