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';
 

tools/ui/src/lib/utils/progress.ts

@@ -0,0 +1,43 @@
+/**
+ * Model load progress helpers for the /models/sse surfaces
+ * (selector row and chat message).
+ */
+
+import { MODEL_LOAD_STAGE_LABELS, MODEL_LOAD_TAIL_SHARE } from '$lib/constants';
+
+/**
+ * Human label for a model load stage.
+ */
+export function modelLoadStageLabel(stage: ApiModelLoadStage): string {
+	return MODEL_LOAD_STAGE_LABELS[stage];
+}
+
+/**
+ * Overall load fraction (0.0 -> 1.0) across the declared stage plan.
+ * text_model fills [0, 1 - tail], each later phase owns one tail slice.
+ */
+export function modelLoadFraction(progress: ModelLoadProgress | null): number {
+	if (!progress) return 0;
+
+	const { stages, current, value } = progress;
+	const tailCount = Math.max(stages.length - 1, 0);
+	const textCeiling = 1 - tailCount * MODEL_LOAD_TAIL_SHARE;
+	const idx = stages.indexOf(current);
+
+	if (idx <= 0) {
+		return value * textCeiling;
+	}
+
+	return textCeiling + (idx - 1 + value) * MODEL_LOAD_TAIL_SHARE;
+}
+
+/**
+ * Single line describing load progress: active stage label and overall percent.
+ * Returns null when there is no progress to show.
+ */
+export function modelLoadProgressText(progress: ModelLoadProgress | null): string | null {
+	if (!progress) return null;
+
+	const label = modelLoadStageLabel(progress.current);
+	return `${label} ${Math.round(modelLoadFraction(progress) * 100)}%`;
+}