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>

Production-ready rewrite: UI kit integration, admin backups, sorting, profile editing, and security hardening ...

- Frontend: complete rewrite using gnexus-ui-kit components (GnNavigationShell,
  GnTable, GnTimeline, GnModal, GnToastProvider, GnSkeleton, etc.)
- Frontend: browser route sync (/secrets/:id, /audit/:id), pagination,
  real-time search debounce, skeleton loading states, retry with backoff
- Frontend: profile editing modal, token creation with scope selection,
  import/export, settings metrics, admin users table
- Backend: backup/restore endpoints for PostgreSQL and SQLite via pg_dump/psql
- Backend: admin backup CRUD, file download endpoint
- Backend: stats endpoint, PATCH /me profile endpoint, CORS middleware,
  graceful shutdown logging
- Backend: table sorting (sort_by/sort_dir) in list_secrets
- Backend: OAuth return_to validation, rate limiting on sensitive endpoints
- MCP: validation fixes, tool description improvements, protocol cleanup
- Tests: 44 passing including new backup test suite
- Docker: add postgresql-client, backups volume, deployment docs
- Docs: backup/restore instructions in deployment guide

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Implement initial gnexus-creds MVP scaffold