fix(ui): Phase 7 C3 修复 — AboutDialog 改用 Rectangle overlay + backdrop

原 AboutDialog inherits Slint Dialog（PopupWindow 子类）并 inline 实例化，没经过运行时 show()，close() 是空操作 → 用户点 Close 没反应、UI 卡死。\n- dialogs.slint：AboutDialog / ConfirmDialog 改为 inherits Rectangle，手写 backdrop（点击关闭）+ content card（Close 按钮调 close-dialog 回调）\n- app.slint：新增 show-about bool 属性；Sidebar About 点击同时触发 about-clicked + show-about=true；最外层 if root.show-about: AboutDialog 渲染\n- 删掉 selection==About 的右栏 marker（现在 About 是模态，不再需要）

ui/app.slint

@@ -53,6 +53,16 @@ export component MainWindow inherits Window {
 
     in-out property <SidebarSelection> selection: SidebarSelection.All;
 
+    // ── Phase 7 C3 fix: AboutDialog overlay. The previous version
+    // inherited Slint's `Dialog` (a PopupWindow-derived type) and
+    // instantiated it inline; without a runtime `.show()` call,
+    // close() was a no-op and the Close button became inert. We
+    // now drive the dialog with a plain boolean (toggled from Rust)
+    // and render it as a manual Rectangle overlay in `if
+    // root.show-about: AboutDialog { … }`. See dialogs.slint. ──
+    in-out property <bool> show-about: false;
+    callback close-about();
+
     // ── Phase 2 C2: mock messages + selection ────────────────────
     in-out property <[MessageItem]> messages;
     in-out property <int> selected-message-index: -1;
@@ -225,6 +235,13 @@ export component MainWindow inherits Window {
                     clicked => {
                         root.selection = SidebarSelection.About;
                         root.about-clicked();
+                        // Phase 7 C3 fix: surface the (now
+                        // Rectangle-overlay) About dialog. The
+                        // Rust `about-clicked` callback still runs
+                        // for telemetry; the actual show happens
+                        // here so the dialog opens the moment the
+                        // user clicks the nav row.
+                        root.show-about = true;
                     }
                 }
             }
@@ -415,21 +432,20 @@ export component MainWindow inherits Window {
                 }
             }
 
-            // About entry stays as a simple right-pane marker; the
-            // modal AboutDialog overlay is wired through AboutDialog in
-            // this component (visibility toggled via `about-clicked`).
-            if root.selection == SidebarSelection.About: Rectangle {
-                background: bg;
-                Text {
-                    x: 24px;
-                    y: 24px;
-                    text: "About — see the dialog.";
-                    color: text-muted;
-                    font-size: 13px;
-                }
-            }
+            // Phase 7 C3 fix: the About page is no longer a separate
+            // right-pane marker. Clicking the sidebar About row opens
+            // a modal dialog overlay (see `if root.show-about:
+            // AboutDialog { … }` further down), so there's nothing
+            // to show in the right pane for `selection == About`.
         }
     }
 
-    AboutDialog {}
+    // Phase 7 C3 fix: conditional About overlay driven by
+    // `show-about`. See the comment on the property above for why
+    // we no longer use Slint's `Dialog` type here.
+    if root.show-about: AboutDialog {
+        close-dialog => {
+            root.show-about = false;
+        }
+    }
 }