ui: fix thinking menu never appearing in single-model mode (#25637)

ui: fix thinking menu never appearing in single-model mode (#25637)

In MODEL mode, modelPropsCache is never populated: fetchModelProps
call sites are gated on router-only state (isRouterMode checks,
routerModels always empty), so supportsThinking always reads an
empty chat template once a model is auto-selected.

Read serverStore.props.chat_template directly in non-router mode,
since the global /props already describes the single loaded model.