agent page: show resolved model as the model badge's tooltip

mara: 'i cannot see if sonnet became claude-sonnet-5 or something
else.' /api/state now carries resolved_model (bus.last_resolved_model(),
already used by serve_common.rs's turn-stats rollup for the same
alias-vs-actual reason) and the model badge shows it as a title
tooltip, same pattern as the ctx/cost badges.

frontend/packages/agent/src/Root.tsx

@@ -192,6 +192,7 @@ export function Root() {
           stateTone={turnDef.tone}
           stateTooltip={STATE_TOOLTIPS[effectiveTurnState]}
           model={state.model}
+          resolvedModel={state.resolved_model ?? undefined}
           availableModels={state.available_models}
           onSelectModel={(name) => {
             postModel(name).then(() => refresh());