feat(chat): make the key-verify lock clickable with a fingerprint dialog

The header key-verify lock was an unexplained hover-only icon. It's now
clickable, opening a dialog that shows the contact's identity-key fingerprint
and explains comparing it out-of-band, with a Copy-key button and a clearer
tooltip. The crypto/verification logic is unchanged (the wording is careful that
the lock means "encrypted to this key", since there is no trusted/verified flag).

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

src/ui/windows/chat_tab.cpp

@@ -106,6 +106,8 @@ std::string s_delete_name;           // its peer name (for the confirm copy / bl
 bool s_show_blocked = false;         // blocked-conversations manager overlay (unblock)
 bool s_show_export_confirm = false;  // "export as plaintext" confirm overlay
 std::string s_export_cid;            // conversation targeted by the export confirm
+bool s_show_fingerprint = false;     // key-fingerprint verification dialog
+std::string s_fingerprint_cid;       // conversation targeted by the fingerprint dialog
 
 // Export file path for a conversation (the same scheme the inline export used).
 static std::string chatExportPathFor(const std::string& peerName) {
@@ -1324,13 +1326,22 @@ void RenderChatTab(App* app)
             // same row. Hidden while renaming (the field takes the whole span).
             if (hasAddr && !renaming) {
                 float ax = textX + std::min(nameW, nameAvail) + nameSpacing;
-                // Key-verify lock — we hold the peer's identity key; hover shows a comparable fingerprint.
+                // Key-verify lock — we hold the peer's identity key; now clickable to open a dialog with the
+                // fingerprint for out-of-band comparison (the old hover-only lock was unexplained).
                 if (showLock) {
                     const float ly = hp.y + (rowH - icoSz) * 0.5f;
+                    ImGui::SetCursorScreenPos(ImVec2(ax, ly));
+                    const bool lockClicked = ImGui::InvisibleButton("##hdr_keylock",
+                                                                    ImVec2(std::max(1.0f, lockW - 5.0f * hdpi), icoSz));
+                    const bool lockHov = ImGui::IsItemHovered();
                     hdl->AddText(icoFont, icoSz, ImVec2(ax, ly),
-                                 material::WithAlpha(material::Primary(), 220), ICON_MD_LOCK);
+                                 lockHov ? material::Primary() : material::WithAlpha(material::Primary(), 220),
-                    if (ImGui::IsMouseHoveringRect(ImVec2(ax, ly), ImVec2(ax + lockW, ly + icoSz)))
+                                 ICON_MD_LOCK);
-                        material::Tooltip("%s\n%s", TR("chat_verify_key"), keyFingerprint(sel->peerPubKey).c_str());
+                    if (lockHov) {
+                        ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
+                        material::Tooltip("%s", TR("chat_verify_tip"));
+                    }
+                    if (lockClicked) { s_fingerprint_cid = sel->cid; s_show_fingerprint = true; }
                     ax += lockW;
                 }
                 // Click-to-copy compact address.
@@ -2207,6 +2218,53 @@ void RenderChatTab(App* app)
         }
     }
 
+    // ---- Key-fingerprint verification dialog ----
+    if (s_show_fingerprint) {
+        const ConvSummary* fp = nullptr;
+        for (const auto& c : convs) if (c.cid == s_fingerprint_cid) { fp = &c; break; }
+        if (!fp || fp->peerPubKey.empty()) {
+            s_show_fingerprint = false;
+            s_fingerprint_cid.clear();
+        } else {
+            material::OverlayDialogSpec ov;
+            ov.title = TR("chat_verify_title");
+            ov.p_open = &s_show_fingerprint;
+            ov.style = material::OverlayStyle::BlurFloat;
+            ov.cardWidth = 500.0f;
+            ov.idSuffix = "chatfp";
+            if (material::BeginOverlayDialog(ov)) {
+                const float fdp = Layout::dpiScale();
+                const std::string fpStr = keyFingerprint(fp->peerPubKey);
+                ImGui::PushTextWrapPos(0.0f);
+                ImGui::PushStyleColor(ImGuiCol_Text, ImGui::ColorConvertU32ToFloat4(material::OnSurfaceMedium()));
+                ImGui::TextWrapped("%s", TR("chat_verify_body"));
+                ImGui::PopStyleColor();
+                ImGui::Spacing();
+                ImGui::TextWrapped("%s", fp->peerName.c_str());
+                ImGui::PushStyleColor(ImGuiCol_Text, ImGui::ColorConvertU32ToFloat4(material::Primary()));
+                ImGui::TextWrapped("%s", fpStr.c_str());
+                ImGui::PopStyleColor();
+                ImGui::PopTextWrapPos();
+                ImGui::Dummy(ImVec2(0, Layout::spacingMd()));
+
+                const float fgap = ImGui::GetStyle().ItemSpacing.x;
+                float wCopy = std::max(110.0f * fdp, ImGui::CalcTextSize(TR("chat_verify_copy")).x + 24.0f * fdp);
+                float wClose = std::max(90.0f * fdp, ImGui::CalcTextSize(TR("chat_cancel")).x + 24.0f * fdp);
+                material::BeginOverlayDialogFooter(wCopy + wClose + fgap, false);
+                if (material::TactileButton(TR("chat_verify_copy"), ImVec2(wCopy, 0))) {
+                    ImGui::SetClipboardText(fp->peerPubKey.c_str());
+                    Notifications::instance().success(TR("copied"));
+                }
+                ImGui::SameLine(0, fgap);
+                if (material::TactileButton(TR("chat_cancel"), ImVec2(wClose, 0))) {
+                    s_show_fingerprint = false;
+                    s_fingerprint_cid.clear();
+                }
+                material::EndOverlayDialog();
+            }
+        }
+    }
+
     // ---- Delete-conversation confirm (revive-on-new-message vs delete & block) ----
     if (s_show_delete_confirm) {
         const float dp = Layout::dpiScale();