feat(ux): clearer chat delivery, explorer empty/range states, honest settings label

Three next-batch UX-audit fixes:
- Chat delivery: the sending/sent glyph has higher contrast and a tooltip
  ("Sent to the network — arrives after it confirms", so a check isn't mistaken
  for "delivered"); a failed send now carries the real reason (ChatMessage::
  fail_reason, set from onChatBroadcastResult via friendlyRpcError, in-memory
  only) in the "not sent" hover tooltip.
- Explorer: a block height beyond the tip says so ("Block N is beyond the chain
  tip (M)") instead of silently snapping to the top; the empty block list shows
  Not connected / Syncing (N behind) / Loading instead of an endless "Loading".
- Settings: the misleading "Reset to Defaults" button (which only reloads the
  last-saved values) is relabeled "Discard unsaved changes" with accurate copy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/chat/chat_service.cpp

@@ -189,8 +189,8 @@ bool ChatService::recordOutgoingPending(const ChatMessage& message) {
     return appended;
 }
 
-void ChatService::resolveOutgoing(const std::string& txid, ChatDelivery delivery) {
-    const ChatMessage* updated = store_.updateDelivery(txid, delivery);
+void ChatService::resolveOutgoing(const std::string& txid, ChatDelivery delivery, const std::string& reason) {
+    const ChatMessage* updated = store_.updateDelivery(txid, delivery, reason);
     if (updated && db_) db_->upsert(*updated);  // overwrite the Sending row with the final status
 }