fix: restore authentik trusted origin and remove auth card U-border shadow

- Auto-derive trusted origin from AUTHENTIK_ISSUER URL so OAuth callbacks
  are accepted without requiring a separate AUTHENTIK_ORIGIN env var
- Remove leftover ssoProvider schema mapping (no longer used with genericOAuth)
- Remove dead @better-auth/sso dependency from package.json
- Drop md:shadow-2xl/md:shadow-lg from auth cards — the downward box-shadow
  was rendering as a U-shaped border (bottom+sides, no top); border +
  backdrop-blur-xl provides sufficient visual separation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -25,7 +25,6 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
-    "@better-auth/sso": "^1.4.12",
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/modifiers": "^9.0.0",
     "@dnd-kit/sortable": "^10.0.0",