feat: Add proxy authentication support for reverse proxy SSO

[juchong]

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Description

Add header-based authentication for integration with identity providers like Authentik, Authelia, OAuth2 Proxy, and Traefik Forward Auth.

Features:

• Extract authenticated username from configurable HTTP header
• Auto-create users on first login (optional)
• Redirect to external logout URL on sign out
• Skip setup page when first user is created via proxy auth
• Show proxy auth status in Security settings UI

Configuration:

• DOCKGE_AUTH_PROXY_HEADER: Header name (e.g., Remote-User)
• DOCKGE_AUTH_PROXY_AUTO_CREATE: Auto-create users (true/false)
• DOCKGE_AUTH_PROXY_LOGOUT_URL: External logout redirect URL

Files:

• backend/proxy-auth.ts: New proxy authentication handler
• backend/dockge-server.ts: Integrate proxy auth on socket connection
• backend/util-server.ts: Add config interfaces
• frontend/src/mixins/socket.ts: Handle proxy auth state
• frontend/src/components/Login.vue: Show proxy auth errors
• frontend/src/components/settings/Security.vue: Proxy auth UI
• frontend/src/lang/en.json: Translation strings
• README.md: Updated documentation with examples
• compose.yaml: Example updated with new environment variables

Discussion(s):

• OAuth2 #797

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)