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.

tests/hermes_cli/test_inventory_pricing.py

@@ -0,0 +1,98 @@
+"""Tests for inventory._apply_pricing — the pricing/tier enrichment that
+
+feeds the desktop GUI model picker (and onboarding) so it can show $/Mtok
+columns + Free/Pro badges and gate paid models on free Nous accounts, the
+same way the `hermes model` CLI picker does.
+"""
+
+import hermes_cli.inventory as inv
+import hermes_cli.models as models_mod
+
+
+def _patch_pricing(monkeypatch, *, free_tier, pricing, unavailable=None):
+    monkeypatch.setattr(models_mod, "get_pricing_for_provider", lambda slug, **kw: pricing.get(slug, {}))
+    monkeypatch.setattr(models_mod, "check_nous_free_tier", lambda *, force_fresh=False: free_tier)
+    monkeypatch.setattr(
+        models_mod, "partition_nous_models_by_tier",
+        lambda ids, pr, free_tier: (
+            [m for m in ids if m not in (unavailable or [])],
+            list(unavailable or []),
+        ),
+    )
+
+
+def test_apply_pricing_formats_per_model_prices(monkeypatch):
+    """Each model gets formatted input/output/cache + a free flag."""
+    _patch_pricing(
+        monkeypatch,
+        free_tier=False,
+        pricing={
+            "openrouter": {
+                "a/paid": {"prompt": "0.000003", "completion": "0.000015", "input_cache_read": "0.0000003"},
+                "b/free": {"prompt": "0", "completion": "0"},
+            }
+        },
+    )
+    rows = [{"slug": "openrouter", "models": ["a/paid", "b/free"]}]
+    inv._apply_pricing(rows)
+
+    pricing = rows[0]["pricing"]
+    assert pricing["a/paid"] == {"input": "$3.00", "output": "$15.00", "cache": "$0.30", "free": False}
+    assert pricing["b/free"]["free"] is True
+    assert pricing["b/free"]["input"] == "free"
+
+
+def test_apply_pricing_nous_free_tier_gates_paid_models(monkeypatch):
+    """A free-tier Nous account marks paid models unavailable and sets the flag."""
+    _patch_pricing(
+        monkeypatch,
+        free_tier=True,
+        pricing={
+            "nous": {
+                "free/model": {"prompt": "0", "completion": "0"},
+                "paid/model": {"prompt": "0.000005", "completion": "0.00001"},
+            }
+        },
+        unavailable=["paid/model"],
+    )
+    rows = [{"slug": "nous", "models": ["free/model", "paid/model"]}]
+    inv._apply_pricing(rows)
+
+    assert rows[0]["free_tier"] is True
+    assert rows[0]["unavailable_models"] == ["paid/model"]
+    assert rows[0]["pricing"]["free/model"]["free"] is True
+
+
+def test_apply_pricing_nous_paid_tier_no_gating(monkeypatch):
+    """A paid Nous account gates nothing."""
+    _patch_pricing(
+        monkeypatch,
+        free_tier=False,
+        pricing={"nous": {"x/model": {"prompt": "0.000001", "completion": "0.000002"}}},
+    )
+    rows = [{"slug": "nous", "models": ["x/model"]}]
+    inv._apply_pricing(rows)
+
+    assert rows[0]["free_tier"] is False
+    assert rows[0]["unavailable_models"] == []
+
+
+def test_apply_pricing_skips_providers_without_pricing(monkeypatch):
+    """A provider with no live pricing simply gets no pricing key."""
+    _patch_pricing(monkeypatch, free_tier=False, pricing={})
+    rows = [{"slug": "anthropic", "models": ["claude-x"]}]
+    inv._apply_pricing(rows)
+
+    assert "pricing" not in rows[0]
+
+
+def test_apply_pricing_failure_is_swallowed(monkeypatch):
+    """A pricing fetch that raises must not break the whole payload."""
+    def boom(slug, **kw):
+        raise RuntimeError("network down")
+
+    monkeypatch.setattr(models_mod, "get_pricing_for_provider", boom)
+    rows = [{"slug": "openrouter", "models": ["a/b"]}]
+    inv._apply_pricing(rows)  # must not raise
+
+    assert "pricing" not in rows[0]