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/models/custom_page.py

@@ -25,6 +25,7 @@ class CustomPage(Base):
     login_submit_selector: Mapped[Optional[str]] = mapped_column(String(512), nullable=True)
     login_autofill_enabled: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     login_autofill_backfilled_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True)
+    linked_upstream_id: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.now(timezone.utc))
     updated_at: Mapped[datetime] = mapped_column(
         DateTime,