Fix Google Flights swap button layout

[hqhq1025]

Summary

Fixes the Google Flights homepage search form layout so the swap button stays centered between the origin and destination fields instead of drifting into the destination field when the origin input has content.

Changes:

• Wraps origin, swap, and destination controls in a dedicated .search-route-fields container.
• Positions .search-swap relative to that route-field group instead of the full search row that also includes date fields.
• Gives route and date groups stable flex sizing and min-width: 0 constraints.
• Keeps the swap button hidden in the mobile stacked layout.

Verification

• python3 -m py_compile sites/google_flights/app.py
• git diff --check
• Playwright render check on the Google Flights homepage:
  • filled origin with LHR
  • verified swap button center equals the seam between origin and destination fields
  • verified swap button is before the dates group, not inside destination/date fields
  • verified mobile layout hides the swap button
• Screenshot artifact: /tmp/google-flights-swap-desktop.png

Fixes #20.