ui: model status and load progress via /models/sse feed (#24878)

* ui: model status and load progress via /models/sse feed

* ui: centralize SSE wire-format delimiters into shared constants for the chat and /models/sse parsers

* ui: type /models/sse event names as a ServerModelsSseEventType enum

Address review from allozaur

tools/ui/src/lib/constants/index.ts

@@ -37,6 +37,8 @@ export * from './mcp-form';
 export * from './mcp-resource';
 export * from './message-export';
 export * from './model-id';
+export * from './model-loading';
+export * from './sse';
 export * from './precision';
 export * from './processing-info';
 export * from './pwa';