Adopt shield keyhole logo, serve dist public assets from SPA fallback ...

gnexus-creds now has its own mark: a shield with a keyhole, drawn in the
gnexus-ui-kit language (dark tile, line-art strokes, square keyhole
terminal in the role color). Ship it as frontend/public/logo.svg, use it
in the navigation shell instead of the external gnexus.space PNG, and
register it as the SVG favicon.

The SPA fallback in main.py returned index.html for every unknown path,
so /logo.svg rendered the HTML shell instead of the image; serve real
files that exist in frontend/dist first and fall back only afterwards.
Also drop two unused imports flagged by ruff.

Co-Authored-By: Claude Code <noreply@anthropic.com>