feat(dashboard): configure main + auxiliary models from Models page (#17802)

Dashboard Models page was analytics-only — no way to pick a model as main
for new sessions or override an auxiliary task slot without hand-editing
config.yaml or running a /model slash command inside a chat.

Changes:
- hermes_cli/web_server.py: three REST endpoints (GET /api/model/options,
  GET /api/model/auxiliary, POST /api/model/set). Reuses
  list_authenticated_providers() from model_switch.py so the REST path
  surfaces the same curated model lists as the TUI-gateway model.options
  JSON-RPC. POST /api/model/set writes model.provider + model.default for
  scope=main, and auxiliary.<task>.{provider,model} for scope=auxiliary
  (with task="" meaning 'all 8 slots' and task="__reset__" resetting them
  to auto).
- web/src/components/ModelPickerDialog.tsx: accepts an optional loader +
  onApply pair so it works without an open chat PTY. ChatSidebar's
  gw-WebSocket path still works unchanged (back-compat).
- web/src/pages/ModelsPage.tsx: Model Settings panel at the top showing
  main model + collapsible list of 8 auxiliary tasks with per-row Change
  buttons and Reset all to auto. Every existing model card gets a
  'Use as' dropdown for one-click assignment to main or any aux slot.
  Cards badged 'main' or 'aux · <task>' when currently assigned.
- website/docs/user-guide/configuring-models.md: new docs page walking
  through both UI paths, aux task override patterns, troubleshooting,
  plus REST/CLI alternatives.
- Screenshots under website/static/img/docs/dashboard-models/.

Applies to new sessions only — running sessions keep their model (use
/model slash command to hot-swap a live session). No prompt-cache
invalidation on existing sessions.

web/src/lib/api.ts

@@ -69,6 +69,14 @@ export const api = {
   getDefaults: () => fetchJSON<Record<string, unknown>>("/api/config/defaults"),
   getSchema: () => fetchJSON<{ fields: Record<string, unknown>; category_order: string[] }>("/api/config/schema"),
   getModelInfo: () => fetchJSON<ModelInfoResponse>("/api/model/info"),
+  getModelOptions: () => fetchJSON<ModelOptionsResponse>("/api/model/options"),
+  getAuxiliaryModels: () => fetchJSON<AuxiliaryModelsResponse>("/api/model/auxiliary"),
+  setModelAssignment: (body: ModelAssignmentRequest) =>
+    fetchJSON<ModelAssignmentResponse>("/api/model/set", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify(body),
+    }),
   saveConfig: (config: Record<string, unknown>) =>
     fetchJSON<{ ok: boolean }>("/api/config", {
       method: "PUT",
@@ -473,6 +481,54 @@ export interface ModelInfoResponse {
   };
 }
 
+// ── Model options / assignment types ──────────────────────────────────
+
+export interface ModelOptionProvider {
+  name: string;
+  slug: string;
+  models?: string[];
+  total_models?: number;
+  is_current?: boolean;
+  is_user_defined?: boolean;
+  source?: string;
+  warning?: string;
+}
+
+export interface ModelOptionsResponse {
+  model?: string;
+  provider?: string;
+  providers?: ModelOptionProvider[];
+}
+
+export interface AuxiliaryTaskAssignment {
+  task: string;
+  provider: string;
+  model: string;
+  base_url: string;
+}
+
+export interface AuxiliaryModelsResponse {
+  tasks: AuxiliaryTaskAssignment[];
+  main: { provider: string; model: string };
+}
+
+export interface ModelAssignmentRequest {
+  scope: "main" | "auxiliary";
+  provider: string;
+  model: string;
+  /** For auxiliary: task slot name, "" for all, "__reset__" to reset all. */
+  task?: string;
+}
+
+export interface ModelAssignmentResponse {
+  ok: boolean;
+  scope?: string;
+  provider?: string;
+  model?: string;
+  tasks?: string[];
+  reset?: boolean;
+}
+
 // ── OAuth provider types ────────────────────────────────────────────────
 
 export interface OAuthProviderStatus {