feat: one-click upstream auth refresh from custom page viewer

- Add linked_upstream_id to CustomPage model with DB migration
- New POST /api/custom-pages/{pid}/refresh-auth endpoint extracts
  credentials from active remote browser and updates linked upstream
- PageViewer toolbar shows key icon button when page has linked upstream
- CustomPages form adds upstream dropdown for remote_browser pages
- Auth capture extracts New-Api-User from localStorage uid/user/self API
- Upstream client sends New-Api-User header in cookie auth mode
- Fix auth capture dialog: transparent background, field persistence,
  login URL defaults to base_url/login, focus on click for keyboard input
- Fix upstream test ASCII encoding with non-header characters validation

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/database.py

@@ -64,4 +64,6 @@ def _migrate_custom_pages():
                     "AND NULLIF(TRIM(login_password), '') IS NOT NULL"
                 )
             )
+        if "linked_upstream_id" not in columns:
+            conn.execute(text("ALTER TABLE custom_pages ADD COLUMN linked_upstream_id INTEGER"))