Implement guard proxy management flow

[bihius]

Summary

This PR migrates the Guard Proxy frontend toward a shadcn/ui-based design system and refreshes the main management flows.

Changes include:

• add shadcn/ui configuration and base UI primitives (button, card, dialog, input, table, badge, alert, and related helpers)
• add Radix/shadcn support dependencies and cn utility wiring
• migrate shared components such as DataTable, Modal, SectionCard, StatCard, status/loading/error/empty states, and page headers
• redesign the app shell/navigation and the login, dashboard, vhosts, policies, policy detail, logs, and not-found pages
• update vhost/policy dialogs and runtime/apply-config UI to use the new component primitives
• refresh Tailwind/theme tokens and keep Dark Reader compatibility tests aligned with the new CSS structure

Review focus

Please focus review on:

• visual consistency across dashboard, logs, vhosts, and policies
• form/dialog behavior after the Radix dialog migration
• table density, empty/error/loading states, and responsive layout
• whether the new token names preserve both existing themes well enough for the MVP

Testing

GitHub CI:

• backend check passed
• frontend check passed
• GitGuardian Security Checks passed

Manual follow-up recommended before merge:

• browser smoke test for login, dashboard, vhost CRUD, policy CRUD, logs filters, and log detail modal
• quick pass in both emerald and frost themes
• mobile-width pass for nav, tables, dialogs, and LogsPage filters

Notes

This is a frontend-wide visual migration. It intentionally keeps domain behavior and API contracts unchanged while replacing the presentation layer with reusable shadcn-style primitives.