feat(chat,app): Model view in sidebar + split key/model selectors

- move the model picker out of the chat header into a dedicated "Model" sidebar
  view; sidebar now seeds two default panels (Model on top, Extensions below)
- split the single model dropdown into two stacked selects: a key selector
  (distinct credential keys) + a model selector (models under the current key)
- pure model-select helpers (splitModelName/joinModelName/modelKeys/modelsForKey),
  split on the FIRST slash so multi-slash model names stay intact
- onSelect still emits the full `<key>/<model>` string (ChatRequest.model unchanged)

1 files changed, 49 insertions, 0 deletions

diff --git a/src/features/chat/model-select.ts b/src/features/chat/model-select.ts
new file mode 100644
index 0000000..b1d70b9
--- /dev/null
+++ b/src/features/chat/model-select.ts
@@ -0,0 +1,49 @@
+/**
+ * Pure helpers for the two-step model picker.
+ *
+ * Models arrive from `GET /models` as `<key>/<model>` strings, where `key` is
+ * the credential name (the part before the FIRST slash) and `model` is the rest.
+ * These pure functions split that into a key selector + a model selector and
+ * recombine the choice — zero DOM, zero Svelte.
+ */
+
+export interface SplitModel {
+	readonly key: string;
+	readonly model: string;
+}
+
+/** Split `<key>/<model>` on the first slash. A slashless name is all-key. */
+export function splitModelName(full: string): SplitModel {
+	const i = full.indexOf("/");
+	if (i === -1) return { key: full, model: "" };
+	return { key: full.slice(0, i), model: full.slice(i + 1) };
+}
+
+/** Recombine a key + model into a `<key>/<model>` name (key-only if no model). */
+export function joinModelName(key: string, model: string): string {
+	return model === "" ? key : `${key}/${model}`;
+}
+
+/** Distinct keys across all models, in first-seen order. */
+export function modelKeys(models: readonly string[]): string[] {
+	const seen = new Set<string>();
+	const out: string[] = [];
+	for (const full of models) {
+		const { key } = splitModelName(full);
+		if (!seen.has(key)) {
+			seen.add(key);
+			out.push(key);
+		}
+	}
+	return out;
+}
+
+/** The model suffixes available under a given key, in order. */
+export function modelsForKey(models: readonly string[], key: string): string[] {
+	const out: string[] = [];
+	for (const full of models) {
+		const split = splitModelName(full);
+		if (split.key === key) out.push(split.model);
+	}
+	return out;
+}