fix: preserve cross-provider model selection across conversation turns

Two-part fix for the model selector jumping back to the profile default
after the first message when a cross-provider model (e.g. a custom relay
like custom:glm-free-relay) is selected.

Backend (api/routes.py):
- _catalog_has_provider() now matches compound provider IDs (e.g.
  custom:zenmux-relay) whose normalized form (custom) is a raw
  catalog provider_id.  Previously the three existing checks missed
  normalized-vs-raw comparison for compound hints, causing the catalog
  lookup to fail and the fallback branch to silently revert the model
  to the profile default on every non-explicit turn.

Frontend (static/messages.js):
- The explicit_model_pick marker was consumed after the first send(),
  so subsequent sends lost the signal and the server repaired the
  model back to the profile default (#3737 regression).
- Now, when the session has a non-default model from a different
  provider than the profile's active provider, every send() sends
  explicit_model_pick=true so the server honors the user's choice
  across the entire conversation.
- The pending onchange marker is still consumed after the first send
  (unchanged behavior); only the explicit_pick inference is broadened.

api/routes.py

@@ -3196,6 +3196,18 @@ def _catalog_has_provider(
         provider_raw in raw_provider_ids
         or (provider_normalized and provider_normalized in raw_provider_ids)
         or (provider_normalized and provider_normalized in normalized_provider_ids)
+        # Compound provider IDs like "custom:zenmux-relay" normalize to
+        # "custom" which is a raw catalog provider_id.  The three checks
+        # above compare raw-vs-raw and normalized-vs-normalized but miss
+        # normalized-vs-raw for compound hints.  Without this, a
+        # @custom:relay-name:model selection is silently reverted to the
+        # profile default on every non-explicit turn because the catalog
+        # lookup fails and the fallback branch at line ~3932 fires.
+        or (
+            provider_normalized
+            and ":" in provider_raw
+            and provider_normalized in raw_provider_ids
+        )
     )
 
 

static/messages.js

@@ -1267,15 +1267,29 @@ async function send(){
     const _pendingPick=(typeof _readPendingSessionModel==='function')
       ? _readPendingSessionModel(activeSid)
       : null;
-    const _explicitPick=_pendingPick
+    const _pendingPickMatch=_pendingPick
       && _pendingPick.model===_modelState.model
       && String(_pendingPick.model_provider||'')===String(_modelState.model_provider||'');
+    // ── Persisted cross-provider pick (#3737 follow-up) ──
+    // The onchange marker is consumed after the first send, so subsequent sends
+    // lose explicit_model_pick and the server "repairs" the model back to the
+    // profile default.  When the session has a non-default model from a different
+    // provider than the profile's active provider, treat every send as explicit
+    // so the server honors the user's choice across the entire conversation.
+    const _defaultModel=(typeof window!=='undefined' && window._defaultModel)||'';
+    const _activeProvider=(typeof window!=='undefined' && window._activeProvider)||null;
+    const _isCrossProviderPick = _modelState.model
+      && _modelState.model_provider
+      && _defaultModel
+      && _modelState.model !== _defaultModel
+      && String(_modelState.model_provider||'') !== String(_activeProvider||'');
+    const _explicitPick = _pendingPickMatch || _isCrossProviderPick;
     // Consume the pending explicit-pick marker for THIS send only. The marker is
     // recorded on modelSelect.onchange and intentionally kept (not cleared on
     // session-update) so it survives the normal pick→update→send flow; clear it here
     // once read so a later send of an unchanged dropdown isn't treated as an explicit
     // pick. (#3739/#3737, Codex catch)
-    if(_explicitPick && typeof _clearPendingSessionModel==='function') _clearPendingSessionModel(activeSid);
+    if(_pendingPickMatch && typeof _clearPendingSessionModel==='function') _clearPendingSessionModel(activeSid);
     explicitPickForPostStart=_explicitPick;
     const startData=await api('/api/chat/start',{method:'POST',body:JSON.stringify({
       session_id:activeSid,message:msgText,