webui: Add Import/Export of Settings configuration + improve architecture (#22803)
* refactor: Settings keys as constant object keys
* chore: Run `npm audit fix`
* refactor: Settings Sections UI
* feat: Refactor Settings structure and implement import/export logic
* feat: Introduce ROUTES constant and RouterService
* refactor: Consolidate settings definitions into registry
* refactor: Update settings page routing structure
* chore: Migrate hardcoded URLs to use ROUTES and RouterService
* feat: Enhance model selection logic for settings and chat
* chore: Update webui static build
* refactor: Address PR review comments
* fix: Remove unneeded setting
* fix: Re-add missing settings
* fix: Add missing `/slots` proxy for webui dev mode
* chore: Dev-mode logs
* fix: Data binding
* fix: Steering for non-agentic flow