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/utils/index.ts

@@ -44,6 +44,9 @@ export { buildProxiedUrl, buildProxiedHeaders } from './cors-proxy';
 // URL utilities
 export { extractRootDomain, sanitizeExternalUrl } from './url';
 
+// Progress helpers
+export { modelLoadFraction, modelLoadProgressText } from './progress';
+
 // Conversation utilities
 export { createMessageCountMap, getMessageCount } from './conversation-utils';