feat(chat): demo-chat seed for the screenshot sweep (debug)

Add App::seedChatDemoData() + a "Seed demo chat" button in the DEBUG OPTIONS
card (shown only in chat-enabled builds, next to the screenshot sweep). It gives
the Chat tab a demo identity and injects a few sample conversations into the
in-memory store (NOT persisted — no DB attached, gone on restart) so the
screenshot sweep captures the tab's real UI (conversation list, thread bubbles,
composer, and the "waiting for reply" contact-request state) instead of the
empty "unlock" hint. No-op when the chat feature is off.

Verified: Linux + Windows build with chat ON, ctest 100%, hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/pages/settings_page.cpp

@@ -2496,6 +2496,12 @@ void RenderSettingsPage(App* app) {
             ImGui::SameLine();
             if (TactileButton(TR("screenshot_open_dir"), ImVec2(0, 0), S.resolveFont("button")))
                 util::Platform::openFolder(app->screenshotDir());
+            if (chat::hushChatFeatureEnabledAtBuild()) {
+                // Populate the Chat tab with demo conversations so the sweep captures its real UI.
+                ImGui::SameLine();
+                if (TactileButton("Seed demo chat", ImVec2(0, 0), S.resolveFont("button")))
+                    app->seedChatDemoData();
+            }
             ImGui::Dummy(ImVec2(0, Layout::spacingSm()));
             ImGui::Separator();
             ImGui::Dummy(ImVec2(0, Layout::spacingSm()));