fix: show toast when model is switched during active session (#419)

When a user switches the model via the model picker while a session has
existing messages, a toast now informs them: 'Model change takes effect
in your next conversation'. This prevents confusion when the model
dropdown updates visually but the running conversation continues with
the original model.

Implementation: 4-line addition in modelSelect.onchange in boot.js,
after the existing provider-mismatch warning. Checks S.messages.length
(the reliable in-memory array) and guards showToast with typeof.

Synthesized from PRs #516 (armorbreak001), #517 and #518 (cloudyun888).
Placement follows #518's correct boot.js approach. Reference corrected
from S.session.messages to S.messages (always initialized by loadSession).

4 new tests in test_provider_mismatch.py::TestModelSwitchToast.

Co-authored-by: armorbreak001 <armorbreak001@users.noreply.github.com>
Co-authored-by: cloudyun888 <cloudyun888@users.noreply.github.com>

tests/test_provider_mismatch.py

@@ -264,3 +264,50 @@ def test_api_models_includes_active_provider():
         "/api/models response missing 'active_provider' field — "
         "frontend needs this to detect provider mismatches"
     )
+
+
+# ── Model switch toast (#419) ─────────────────────────────────────────────────
+
+class TestModelSwitchToast:
+    """Toast appears when user switches model during an active session."""
+
+    def test_toast_in_model_select_onchange(self):
+        """modelSelect.onchange must show a toast when S.messages is non-empty."""
+        src = _read("static/boot.js")
+        # Find the onchange block
+        idx = src.find("modelSelect').onchange")
+        assert idx != -1, "modelSelect.onchange not found in boot.js"
+        block = src[idx:idx + 1100]
+        assert "Model change takes effect in your next conversation" in block, (
+            "modelSelect.onchange must show a toast when switching model mid-session"
+        )
+
+    def test_toast_guards_on_messages_length(self):
+        """Toast must only fire when there are existing messages (active session)."""
+        src = _read("static/boot.js")
+        idx = src.find("Model change takes effect in your next conversation")
+        assert idx != -1
+        # Look back 200 chars for the S.messages guard
+        surrounding = src[max(0, idx - 200):idx + 50]
+        assert "S.messages" in surrounding and ".length" in surrounding, (
+            "Model switch toast must be gated on S.messages.length > 0"
+        )
+
+    def test_toast_uses_show_toast_not_alert(self):
+        """Toast must use showToast(), not alert()."""
+        src = _read("static/boot.js")
+        idx = src.find("Model change takes effect in your next conversation")
+        assert idx != -1
+        surrounding = src[max(0, idx - 50):idx + 100]
+        assert "showToast" in surrounding, "Must use showToast() not alert()"
+        assert "alert(" not in surrounding, "Must not use alert()"
+
+    def test_toast_has_typeof_showtoast_guard(self):
+        """Toast call must guard typeof showToast to be safe during boot."""
+        src = _read("static/boot.js")
+        idx = src.find("Model change takes effect in your next conversation")
+        assert idx != -1
+        surrounding = src[max(0, idx - 100):idx + 50]
+        assert "typeof showToast" in surrounding, (
+            "showToast call must be guarded with typeof check"
+        )