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>

static/boot.js

@@ -420,6 +420,10 @@ $('modelSelect').onchange=async()=>{
     const warn=_checkProviderMismatch(selectedModel);
     if(warn&&typeof showToast==='function') showToast(warn,4000);
   }
+  // Notify user that model changes only take effect in the next conversation (#419)
+  if(S.messages && S.messages.length > 0 && typeof showToast==='function'){
+    showToast('Model change takes effect in your next conversation', 3000);
+  }
 };
 $('msg').addEventListener('input',()=>{
   autoResize();