feat(desktop): show model pricing + free/paid tier gating in GUI picker

The CLI `hermes model` picker shows per-model $/Mtok pricing and gates paid
models on free Nous accounts. The GUI picker showed bare model names. Bring it
to parity across both the model-picker dialog and onboarding confirm card.

Backend:
- inventory.build_models_payload gains a pricing=True flag → _apply_pricing
  enriches each provider row with formatted per-model pricing
  ({input,output,cache,free}) via the same _format_price_per_mtok the CLI uses,
  and for Nous adds free_tier + unavailable_models (paid models a free user
  can't select) via check_nous_free_tier + partition_nous_models_by_tier.
  Best-effort: any pricing/tier failure is swallowed and fails open (no gating).
- /api/model/options and TUI model.options now pass pricing=True so the
  global picker and in-session picker both carry pricing.

Frontend:
- ModelOptionProvider gains pricing/free_tier/unavailable_models; new
  ModelPricing type.
- model-picker dialog renders In/Out $/Mtok (or a Free pill) per model, a
  Free tier/Pro badge on the Nous heading, and disables + grays unavailable
  paid models for free users with a 'Pro models need a paid subscription' note.
- onboarding confirm card shows the chosen model's price + tier badge.

Tests: test_inventory_pricing covers price formatting, free-tier gating,
paid no-gating, providers without pricing, and swallowed failures.

hermes_cli/web_server.py

@@ -1311,7 +1311,7 @@ def get_model_options():
     try:
         from hermes_cli.inventory import build_models_payload, load_picker_context
 
-        return build_models_payload(load_picker_context(), max_models=50)
+        return build_models_payload(load_picker_context(), max_models=50, pricing=True)
     except Exception:
         _log.exception("GET /api/model/options failed")
         raise HTTPException(status_code=500, detail="Failed to list model options")