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/model-loading.ts

@@ -0,0 +1,14 @@
+/**
+ * Labels shown while a model loads, keyed by the stage reported on /models/sse.
+ */
+export const MODEL_LOAD_STAGE_LABELS: Record<ApiModelLoadStage, string> = {
+	text_model: 'Loading weights',
+	spec_model: 'Loading draft',
+	mmproj_model: 'Loading projector'
+};
+
+/**
+ * Share of the bar reserved for each load phase after text_model.
+ * text_model fills the rest, so a plain model reaches 100% on its own.
+ */
+export const MODEL_LOAD_TAIL_SHARE = 0.1;