feat: skin engine + Twenty CRM theme for iistwin

- Add client/src/themes/ with tokens, built-in skins (iistwin, twenty, omegaprom)
- Add ThemeProvider with data-skin/data-theme and runtime CSS for custom skins
- Add npm run skins:generate script for build-time CSS generation
- Add organization_skins table, migration and CRUD API
- Add activeSkinId to organizations and expose it in /api/auth/me
- Add skin switcher in /profile and admin skin editor in /settings/appearance
- Adapt index.css hardcoded colors to skin tokens
- Update AGENTS.md and Obsidian notes

server/routes/auth.core.routes.ts

@@ -203,7 +203,8 @@ export function registerAuthCoreRoutes(router: Router): void {
             id: user.organization.id,
             name: user.organization.name,
             slug: user.organization.slug,
-            displayName: user.organization.displayName
+            displayName: user.organization.displayName,
+            activeSkinId: user.organization.activeSkinId
           } : null
         }
       });