feat: settings dialog for update config (gear entry)

A low-key "⚙ 配置" button at the right end of the flow-tab row opens a settings
dialog to edit update_source / update_user / update_pass, with a test-connection
button that distinguishes "reachable & up to date" from "cannot connect".

- settings_dialog.py: pure-UI QDialog; password mask + show; test runs off the
  UI thread via load_manifest and reports the result
- update_service: add load_manifest() (raises on failure, unlike check_for_update)
- main_window: gear button, _open_settings persists via save_config and re-runs
  the update check; _update_found connected once to avoid duplicate banners
- tests: +2 for load_manifest

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

src/app/main_window.py

@@ -6,6 +6,7 @@ from pathlib import Path
 from PySide6.QtCore import Qt, QUrl, Signal
 from PySide6.QtGui import QDesktopServices
 from PySide6.QtWidgets import (
+    QDialog,
     QHBoxLayout,
     QLabel,
     QMainWindow,
@@ -26,6 +27,7 @@ from app.widgets.export_panel import ExportPanel
 from app.widgets.image_canvas import ImageCanvas
 from app.widgets.image_list_panel import ImageListPanel
 from app.widgets.queue_panel import QueuePanel
+from app.widgets.settings_dialog import SettingsDialog
 from app.widgets.template_panel import TemplatePanel
 from app.widgets.transform_panel import TransformPanel
 
@@ -108,8 +110,34 @@ class MainWindow(QMainWindow):
         layout.addWidget(self._tab_bar)
         layout.addStretch()
 
+        # Right-aligned utility entry — not a numbered workflow step (docs/07 §4.3)
+        self._settings_btn = QPushButton("⚙ 配置")
+        self._settings_btn.setObjectName("settingsBtn")
+        self._settings_btn.setCursor(Qt.PointingHandCursor)
+        self._settings_btn.clicked.connect(self._open_settings)
+        layout.addWidget(self._settings_btn)
+
         return container
 
+    def _open_settings(self):
+        """Open the settings dialog; persist + re-check on save (docs/07 §4.4)."""
+        dlg = SettingsDialog(
+            self,
+            update_source=self._config.get("update_source", ""),
+            update_user=self._config.get("update_user", ""),
+            update_pass=self._config.get("update_pass", ""),
+            current_version=APP_VERSION,
+        )
+        if dlg.exec() == QDialog.Accepted:
+            self._config.update(dlg.values())
+            save_config(self._config)
+            self._refresh_update_check()
+
+    def _refresh_update_check(self):
+        """Re-run the update check after the source/credentials changed."""
+        self._update_banner.setVisible(False)
+        self._start_update_check()
+
     # ------------------------------------------------------------------
     # Update notification (stage 2: notify-only, see docs/10-lan-update.md)
     # ------------------------------------------------------------------
@@ -117,6 +145,7 @@ class MainWindow(QMainWindow):
     def _create_update_banner(self):
         """A thin info bar shown when a newer version is found. Hidden by default."""
         self._update_info = None
+        self._update_found.connect(self._on_update_found)   # connect once
 
         bar = QWidget()
         bar.setObjectName("updateBanner")
@@ -149,13 +178,16 @@ class MainWindow(QMainWindow):
         return bar
 
     def _start_update_check(self):
-        """Check the configured update source for a newer version, off the UI thread."""
+        """Check the configured update source for a newer version, off the UI thread.
+
+        Safe to call again after the settings change; the result signal is
+        connected once in _create_update_banner, not here.
+        """
         source = self._config.get("update_source", "")
         if not source:
             return
         user = self._config.get("update_user", "")
         password = self._config.get("update_pass", "")
-        self._update_found.connect(self._on_update_found)
 
         def worker():
             try:
@@ -636,6 +668,15 @@ class MainWindow(QMainWindow):
             QTabBar#flowTabBar::tab:disabled {
                 color: #bbbbbb;
             }
+            #settingsBtn {
+                font-family: "Microsoft YaHei", "Segoe UI", sans-serif;
+                font-size: 12px;
+                color: #666666;
+                background: transparent;
+                border: none;
+                padding: 4px 14px;
+            }
+            #settingsBtn:hover { color: #0078d4; background: #e8f0fb; }
 
             /* Work splitter */
             QSplitter#workSplitter::handle {