Refactor detail-header to GnPageHeader and remove obsolete card hover override ...

- Replace custom detail-header div with GnPageHeader component
- Remove custom .detail-header and .detail-title CSS
- Remove card:hover animation override (animations removed from UI kit)

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

Apply gnexus-ui-kit 0.4.0 improvements: GnCopyButton, GnTagInput, GnSwitch, GnConfirmDialog, GnTooltip, GnUserCard ...

- Replace manual copy logic with GnCopyButton in SecretDetailPanel
- Replace comma-separated tag input with GnTagInput (array-based)
- Replace access checkboxes with GnSwitch for better UX
- Replace custom confirm modals with GnConfirmDialog
- Add GnTooltip to reveal/hide/edit action icons
- Replace admin table with GnUserCard grid layout
- Replace error paragraphs with GnAlert components
- Remove copiedFieldKey and copyValue custom logic
- Update form tags handling from string to array

Co-Authored-By: Claude Opus 4.7 <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>

Hide unused tab panels

Compact header search

Remove workspace surface framing

Move search into page header

Split secrets list and detail views

Use table for secrets list

Mask revealed fields by default

Improve modal close and revealed field actions

Remove form grid width cap

Improve create modal density and animation

Reduce card-heavy frontend layout

Use gnexus ui kit across frontend

Align UI with gnexus ui kit components

Improve create secret modal UX

Add admin diagnostics UI

Add Docker packaging and admin user UI

Add UI history audit and import flows

Add UI secret and token management actions

Improve UI layout and serve kit fonts

Implement initial gnexus-creds MVP scaffold