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.h

@@ -40,7 +40,8 @@ public:
 
     // Update an outgoing echo's delivery status by its local txid. Returns a pointer to the updated
     // message (for the caller to persist), or nullptr if no message has that txid.
-    const ChatMessage* updateDelivery(const std::string& txid, ChatDelivery delivery);
+    const ChatMessage* updateDelivery(const std::string& txid, ChatDelivery delivery,
+                                      const std::string& reason = {});
 
     // Distinct conversation ids, in first-seen order.
     std::vector<std::string> conversationIds() const;