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

migrations/0054_organization_skins.sql

@@ -0,0 +1,27 @@
+-- Organization skin storage and active skin selector
+CREATE TABLE IF NOT EXISTS organization_skins (
+  id SERIAL PRIMARY KEY,
+  organization_id INTEGER NOT NULL REFERENCES organizations(id) ON DELETE CASCADE,
+  skin_id VARCHAR(50) NOT NULL,
+  name VARCHAR(100) NOT NULL,
+  description TEXT,
+  tokens JSONB NOT NULL,
+  is_editable BOOLEAN DEFAULT true,
+  is_active BOOLEAN DEFAULT false,
+  created_by INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  created_at TIMESTAMP DEFAULT NOW(),
+  updated_at TIMESTAMP DEFAULT NOW(),
+  UNIQUE(organization_id, skin_id)
+);
+
+CREATE INDEX IF NOT EXISTS organization_skins_org_idx ON organization_skins(organization_id);
+
+ALTER TABLE organizations ADD COLUMN IF NOT EXISTS active_skin_id VARCHAR(50) DEFAULT 'iistwin';
+
+-- Tenant isolation
+ALTER TABLE organization_skins ENABLE ROW LEVEL SECURITY;
+DROP POLICY IF EXISTS tenant_iso ON organization_skins;
+CREATE POLICY tenant_iso ON organization_skins USING (
+  current_setting('app.is_superadmin', true) = 'true'
+  OR organization_id = NULLIF(current_setting('app.current_org_id', true), '')::int
+);