[openstreetmap/openstreetmap-website] Add demo links for routing engine directions Fixes #6387 (PR #6693)

[Areeb Siddiqui]

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### Description
Changes:
- Extend the routes variable to include selected routing mode and engine
- Introduce a getDemoLink helper function to generate routing engine demo URLs with appropriate parameters
- Add a demo link to the directions view (directions/show.html.erb) so users can easily open the route in the external routing engine demo
- The changes have been implemented as suggested in [this](https://github.com/openstreetmap/openstreetmap-website/issues/5224#issuecomment-2365144537) comment

### How has this been tested?
Tested locally using the Docker development environment.

- Verified directions page renders correctly for different routing modes and engines
- Confirmed the generated demo link opens the selected routing engine’s demo page with equivalent route parameters
- Ran Rails test suite with system tests skipped
- Ran RuboCop, ESLint, and ERB lint checks and fixed all reported issues

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6693

-- Commit Summary --

  * Add demo links for routing engine directions

-- File Changes --

    M app/assets/javascripts/index/directions-route-output.js (36)
    M app/assets/javascripts/index/directions.js (7)
    M app/views/directions/show.html.erb (3)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6693.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6693.diff

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