fix: clarify update network failures

static/ui.js

@@ -3202,8 +3202,29 @@ function dismissUpdate(){
   const b=$('updateBanner');if(b)b.classList.remove('visible');
   sessionStorage.setItem('hermes-update-dismissed','1');
 }
+function _isUpdateApplyNetworkError(error){
+  const message=(error&&error.message)||String(error||'');
+  return /Failed to fetch|NetworkError|Load failed/i.test(message);
+}
+function _formatUpdateApplyExceptionMessage(error){
+  if(_isUpdateApplyNetworkError(error)){
+    return 'Update failed: could not reach the WebUI server. It may have restarted or the connection was interrupted. Please wait a few seconds, reload the page, then check the server if it still does not come back.';
+  }
+  const message=(error&&error.message)||String(error||'unknown error');
+  return 'Update failed: '+message;
+}
 async function applyUpdates(){
+  if(window._updateApplyInFlight) return;
+  window._updateApplyInFlight=true;
   const btn=$('btnApplyUpdate');
+  const resetApplyButton=(delayMs)=>{
+    const reset=()=>{
+      window._updateApplyInFlight=false;
+      if(btn){btn.disabled=false;btn.textContent='Update Now';}
+    };
+    if(delayMs>0) setTimeout(reset,delayMs);
+    else reset();
+  };
   if(btn){btn.disabled=true;btn.textContent='Updating\u2026';}
   const errEl=$('updateError');
   if(errEl){errEl.style.display='none';errEl.textContent='';}
@@ -3219,7 +3240,7 @@ async function applyUpdates(){
       const res=await api('/api/updates/apply',{method:'POST',body:JSON.stringify({target})});
       if(!res.ok){
         _showUpdateError(target,res);
-        if(btn){btn.disabled=false;btn.textContent='Update Now';}
+        resetApplyButton(0);
         return;
       }
     }
@@ -3228,9 +3249,10 @@ async function applyUpdates(){
     sessionStorage.removeItem('hermes-update-dismissed');
     _waitForServerThenReload();
   }catch(e){
-    if(errEl){errEl.textContent='Update failed: '+e.message;errEl.style.display='block';}
-    else showToast('Update failed: '+e.message);
-    if(btn){btn.disabled=false;btn.textContent='Update Now';}
+    const msg=_formatUpdateApplyExceptionMessage(e);
+    if(errEl){errEl.textContent=msg;errEl.style.display='block';}
+    else showToast(msg);
+    resetApplyButton(_isUpdateApplyNetworkError(e)?5000:0);
   }
 }
 function _showUpdateError(target,res){

tests/test_update_apply_ui.py

@@ -0,0 +1,42 @@
+"""Frontend regression coverage for Update Now apply failures (#1321)."""
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+UI_JS = ROOT / "static" / "ui.js"
+
+
+def _ui_js() -> str:
+    return UI_JS.read_text(encoding="utf-8")
+
+
+def test_update_apply_network_error_has_recovery_message_not_raw_failed_to_fetch():
+    """Network/interrupted update apply failures should not surface raw fetch text alone."""
+    src = _ui_js()
+    assert "function _formatUpdateApplyExceptionMessage" in src
+    assert "could not reach the WebUI server" in src
+    assert "restarted or the connection was interrupted" in src
+    assert "wait a few seconds, reload the page, then check the server" in src
+    assert "Update failed: '+e.message" not in src
+    assert 'Update failed: "+e.message' not in src
+
+
+def test_update_apply_structured_server_errors_still_use_json_message_path():
+    """Server-reachable JSON errors must keep the existing targeted message path."""
+    src = _ui_js()
+    apply_start = src.index("async function applyUpdates()")
+    show_error_call = src.index("_showUpdateError(target,res);", apply_start)
+    reset_button = src.index("resetApplyButton(0);", show_error_call)
+    assert show_error_call < reset_button
+    assert "const msg='Update failed ('+target+'): '+(res.message||'unknown error');" in src
+
+
+def test_update_apply_prevents_duplicate_apply_requests_while_in_flight():
+    """Double-clicks should not send a second update apply request during restart race windows."""
+    src = _ui_js()
+    apply_start = src.index("async function applyUpdates()")
+    next_fn = src.index("function _showUpdateError", apply_start)
+    body = src[apply_start:next_fn]
+    assert "window._updateApplyInFlight" in body
+    assert "if(window._updateApplyInFlight) return;" in body
+    assert "window._updateApplyInFlight=true;" in body
+    assert "window._updateApplyInFlight=false;" in body