feat: add assistant board styling tools

apps/nextjs/src/app/api/assistant/chat/route.ts

@@ -94,6 +94,7 @@ Action rules:
 - A prose response such as "Please confirm if you would like me to proceed", "Would you like me to proceed?", or a parameter summary that asks for confirmation is incorrect. Never ask for a second textual confirmation before an approval-gated tool call.
 - Do not retry a denied action.
 - Before creating an app, call configure_app with the best defaults so the user can review Homarr's native app form. Its icon picker searches Homarr's local icon repository. Use the returned values for app_create.
+- Before changing a board's custom CSS or visual and behavior settings, call board_getBoardSettings to read the current values, then call configure_board_settings with only the requested proposed changes. The user can edit the complete CSS and settings in Homarr's native form. Use its returned flat object for board_savePartialBoardSettings, which then shows the mutation approval. Never overwrite existing CSS rules unless the user explicitly requests replacement.
 - When choosing an app icon without configure_app, call the Homarr icon findIcons tool first and use one of its returned local icon URLs. Never invent a third-party icon CDN URL.
 - Browser tools can navigate within Homarr or open existing Homarr UI. Never navigate to an arbitrary external URL.
 - Keep responses concise and lead with the result. Summarize tool output instead of dumping JSON.