[PM-37804] feat: Confirm before leaving the app to Stripe checkout

[SaintPatrck]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-37804

📔 Objective

The Upgrade now CTA on the Free Premium plan view launched the Stripe checkout URL with no acknowledgement that the user was about to leave the app. Mirror the Manage Plan confirmation pattern: the button now opens a two-button dialog ("Continue to Stripe?" / "You'll go to Stripe's secure checkout to complete your purchase.") and only fetches the checkout URL when the user confirms, giving them a chance to back out without an unnecessary network call.

Also renames the existing stripe_checkout_footer string resource to mirror its value, matching the project convention for naming resources from their text.

📸 Screenshots

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR adds a confirmation dialog before launching the Stripe checkout from the Free Premium plan view, mirroring the existing Manage Plan confirmation pattern in the same screen. The implementation reuses BitwardenTwoButtonDialog with rememberSaveable for state persistence, defers the network call until the user confirms, and renames stripe_checkout_footer to match the project's naming-from-content convention. Test coverage adds three new cases covering dialog display, confirm, and cancel paths — matching the structure of the existing ManagePlanButton tests.

Code Review Details

No findings. The dialog implementation is a direct mirror of the existing PremiumContent confirmation pattern (same file, lines 499 and 542-557), the new string follows the documented snake_case-from-content convention, and the localized stripe_checkout_footer entries will be cleaned up by Crowdin on its next sync — consistent with prior renames in this codebase (e.g., ask_to_add_login → ask_to_add_item in #5758).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.46%. Comparing base (31011b5) to head (16f68d0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6958      +/-   ##
==========================================
+ Coverage   85.49%   86.46%   +0.96%     
==========================================
  Files        1023      870     -153     
  Lines       66231    63464    -2767     
  Branches     9308     9207     -101     
==========================================
- Hits        56623    54872    -1751     
+ Misses       6422     5432     -990     
+ Partials     3186     3160      -26     

Flag	Coverage Δ
app-data	17.04% <0.00%> (+0.18%)	⬆️
app-ui-auth-tools	19.14% <0.00%> (-0.23%)	⬇️
app-ui-platform	16.43% <100.00%> (-0.51%)	⬇️
app-ui-vault	27.89% <0.00%> (-0.58%)	⬇️
authenticator	6.23% <0.00%> (-0.01%)	⬇️
lib-core-network-bridge	4.06% <0.00%> (-0.01%)	⬇️
lib-data-ui	1.15% <0.00%> (-0.01%)	⬇️

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