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/app_network.cpp

@@ -5062,7 +5062,8 @@ void App::onChatBroadcastResult(const LiteInflightOp& op, bool ok, const std::st
                         std::to_string(chat_send_queue_.front().retries) + "); message stays 'Sending'");
         return;                                              // leave at front
     }
-    chat_service_.resolveOutgoing(op.echoLocalId, chat::ChatDelivery::Failed);  // hard failure -> Retry affordance
+    chat_service_.resolveOutgoing(op.echoLocalId, chat::ChatDelivery::Failed,
+                                  util::friendlyRpcError(error).substr(0, 160));  // hard failure -> Retry affordance + reason
     if (frontMatches) chat_send_queue_.pop_front();
     wallet::liteLog("chat: send FAILED: " + error);
 }