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_store.cpp

@@ -61,10 +61,12 @@ std::vector<ChatMessage> ChatStore::conversation(const std::string& conversation
     return out;
 }
 
-const ChatMessage* ChatStore::updateDelivery(const std::string& txid, ChatDelivery delivery) {
+const ChatMessage* ChatStore::updateDelivery(const std::string& txid, ChatDelivery delivery,
+                                             const std::string& reason) {
     for (auto& message : messages_) {
         if (message.txid == txid) {
             message.delivery = delivery;
+            message.fail_reason = (delivery == ChatDelivery::Failed) ? reason : std::string();
             ++revision_;
             return &message;
         }