fix: migrate to tailwind v4 and TS6, add oauth2 dashboard_host setting

- migrate Tailwind v3->v4 (postcss @tailwindcss/postcss, CSS-first @theme/@plugin/@custom-variant)
- fix TS6 build: ignoreDeprecations for baseUrl, react-day-picker v10 classNames, test type fixes
- add dashboard_host setting field with i18n

tsconfig.json

@@ -9,6 +9,7 @@
         }
     ],
     "compilerOptions": {
+        "ignoreDeprecations": "6.0",
         "baseUrl": ".",
         "paths": {
             "@/*": ["./src/*"]