Stage 406: fix conflict-resolution bug — restore sessionModelState definition in _hydrateBootModelDropdown

My earlier conflict resolution between #2716 master and #2726 PR
dropped the 'const sessionModelState=...' assignment that the
.then() callback body uses on 6 different lines (1596, 1600, 1601,
1607, 1608, 1610). Without it boot.js would ReferenceError on every
boot. Caught by tests/test_new_chat_default_model_frontend.py::test_boot_model_hydration_prefers_active_session_over_persisted_model
which I'd missed in the initial touched-tests gate. Adds the
assignment back at the top of the .then() callback — semantically
matches the original #2716 master shape (S.session.model → wrap in
{model,model_provider} object, else null).

static/boot.js

@@ -1587,6 +1587,9 @@ function applyBotName(){
   // options are enough for first paint; the dynamic provider list can settle
   // after the saved session is visible.
   const _hydrateBootModelDropdown=()=>populateModelDropdown({preferProfileDefaultOnFreshBoot:true}).then(()=>{
+    const sessionModelState=S.session&&S.session.model
+      ? {model:S.session.model,model_provider:S.session.model_provider||null}
+      : null;
     const savedState=(typeof _readPersistedModelState==='function')
       ? _readPersistedModelState()
       : (localStorage.getItem('hermes-webui-model')?{model:localStorage.getItem('hermes-webui-model'),model_provider:null}:null);