refactor: remove the non-functional 设置 button from the header

The 设置 button was a layout placeholder from the initial header build —
never wired to anything, so clicking it did nothing. Remove it (and its
style) rather than leave a dead control; current settings (output
dir/format/quality) are editable inline in the export panel.

Docs: UI design 4.1 records that no settings entry is provided for now.

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

src/app/main_window.py

@@ -71,7 +71,7 @@ class MainWindow(QMainWindow):
         self._restore_preferences()
 
     def _create_header(self):
-        """Top header: app name, version label, settings button."""
+        """Top header: app name and version label."""
         header = QWidget()
         header.setObjectName("header")
         header.setFixedHeight(_HEADER_HEIGHT)
@@ -87,14 +87,9 @@ class MainWindow(QMainWindow):
         version.setObjectName("appVersion")
         version.setAlignment(Qt.AlignVCenter)
 
-        settings_btn = QPushButton("设置")
-        settings_btn.setObjectName("settingsBtn")
-        settings_btn.setFixedWidth(60)
-
         row.addWidget(title)
         row.addWidget(version)
         row.addStretch()
-        row.addWidget(settings_btn)
 
         return header
 
@@ -479,14 +474,6 @@ class MainWindow(QMainWindow):
                 font-size: 11px;
                 color: #888888;
             }
-            #settingsBtn {
-                border: 1px solid #d6d6d6;
-                border-radius: 3px;
-                padding: 3px 8px;
-                background: #f7f7f7;
-                font-size: 12px;
-            }
-            #settingsBtn:hover { background: #e8e8e8; }
 
             /* Flow tab bar */
             #tabBarContainer {