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/ui/windows/chat_tab.cpp

@@ -1679,8 +1679,15 @@ void RenderChatTab(App* app)
                         const char* glyph = sending ? ICON_MD_SCHEDULE : ICON_MD_DONE;
                         const float gs = icfSz * 0.82f;
                         const ImVec2 gsz = icf->CalcTextSizeA(gs, FLT_MAX, 0.0f, glyph);
-                        dl->AddText(icf, gs, ImVec2(bmax.x - gsz.x, bmax.y + 1.0f * dp),
-                                    material::WithAlpha(material::OnSurface(), sending ? 95 : 135), glyph);
+                        const ImVec2 gpos(bmax.x - gsz.x, bmax.y + 1.0f * dp);
+                        dl->AddText(icf, gs, gpos,
+                                    material::WithAlpha(material::OnSurface(), sending ? 170 : 200), glyph);
+                        // Explain the glyph on hover — a check means "broadcast to the network", not "delivered".
+                        const float hpad = 4.0f * dp;
+                        if (ImGui::IsWindowHovered() &&
+                            ImGui::IsMouseHoveringRect(ImVec2(gpos.x - hpad, gpos.y - hpad),
+                                                       ImVec2(gpos.x + gsz.x + hpad, gpos.y + gsz.y + hpad)))
+                            material::Tooltip("%s", TR(sending ? "chat_status_sending" : "chat_status_sent"));
                         ImGui::Dummy(ImVec2(0.0f, gs + 2.0f * dp));   // reserve the status glyph (no ItemSpacing now)
                     }
                     // Failed send → right-aligned "not sent" + Retry (kept — it's actionable).
@@ -1692,6 +1699,12 @@ void RenderChatTab(App* app)
                         ImGui::PushFont(metaFont); ImGui::PushStyleColor(ImGuiCol_Text, material::Error());
                         ImGui::TextUnformatted(TR("chat_send_failed"));
                         ImGui::PopStyleColor(); ImGui::PopFont();
+                        if (ImGui::IsItemHovered()) {
+                            std::string tip = m.fail_reason.empty()
+                                ? std::string(TR("chat_failed_generic"))
+                                : std::string(TR("chat_failed_generic")) + "\n" + m.fail_reason;
+                            material::Tooltip("%s", tip.c_str());
+                        }
                         ImGui::SameLine();
                         if (ImGui::SmallButton(TR("chat_retry"))) {
                             // A failed contact request must be re-sent as a request (it has no peer key