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

scripts/dev/generate-skins.ts

@@ -0,0 +1,8 @@
+import { writeFileSync } from 'fs';
+import { generateAllSkinsCss } from '../../client/src/themes/tokens';
+import { BUILT_IN_SKINS } from '../../client/src/themes/index';
+
+const css = generateAllSkinsCss(BUILT_IN_SKINS);
+const header = '/* Auto-generated from client/src/themes/skins/*.ts */\n\n';
+writeFileSync('client/src/themes/styles/generated-skins.css', header + css);
+console.log(`✓ generated-skins.css updated (${BUILT_IN_SKINS.length} skins)`);