Fix back navigation from activity page to cards

[flavorjones]

Summary

• Add root_path to the prefer_referrer allowlist on the card show page so the back link can be rewritten when coming from the activity page
• Switch event card links from HTML target="_top" to data-turbo-frame="_top" so Turbo handles the navigation and turbo:before-visit fires to save the referrer
• Normalize trailing slashes in the referrer path comparison so /account_id and /account_id/ both match

Test plan

• System test: navigate from activity page → card → back link says "Back to Home" and returns to activity
• System test: same flow works when activity URL has no trailing slash
• Existing back link tests still pass (board filter, global filter, non-filter page)

[Copilot]

Pull request overview

This PR improves “back” navigation when opening a card from the Activity (Home) page by ensuring Turbo-driven navigation captures the referrer and that referrer matching is robust to trailing-slash differences.

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Changes:

• Allow card back-link referrer rewriting when the referrer is the Activity (root) page.
• Make Activity event card links Turbo-handled (data-turbo-frame="_top") so turbo:before-visit fires and the referrer is saved.
• Normalize trailing slashes when comparing allowed referrer paths vs. the stored referrer.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/system/back_link_navigation_test.rb	Adds system coverage for Activity → Card → Back (including no-trailing-slash Activity URL).
app/views/events/event/_layout.html.erb	Updates event card links to use Turbo frame navigation at the top-level.
app/views/cards/show.html.erb	Adds root_path to the back-link referrer allowlist for card show.
app/javascript/controllers/turbo_navigation_controller.js	Normalizes trailing slashes during allowed-path vs referrer-path comparison.

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