Fix custom system-messages not applied to auto-generated nav buttons

[Hergie]

I did not manage to set a custom label for automatically generated next buttons (I wanted to translate these into Finnish). Claude's solution below.

Summary

• Bug 1: extract_html_pages() reads settings_yaml$system_messages (underscore) but the YAML key is system-messages
  (hyphen). This causes custom message overrides (e.g., translating "Next" to another language) to be silently ignored for
  auto-generated navigation buttons. Fixed by using backtick-quoted `system-messages`.

• Bug 2: survey_files_need_updating() does not check whether _survey/settings.yml is newer than pages.rds. Since
  create_settings_yaml() runs in sd_ui() and may update settings.yml (e.g., when system-messages are added to the YAML header), pages.rds must be regenerated to pick up the new button labels. Without this check, users must manually delete pages.rds to see updated nav button text.

Reproduction

1. Add a system-messages: block as a top-level YAML key in survey.qmd with e.g. next: "Seuraava"
2. Run shiny::runApp()
3. The console shows "Custom system-messages entries found" but the Next button still shows "Next"

Solution

Two changes in R/config.R:

1. Line 1412: Changed settings_yaml$system_messages to settings_yaml$´system-messages´ so extract_html_pages()
   correctly reads the kebab-case YAML key.
2. Line 355–362: Added settings.yml timestamp to the staleness check in survey_files_need_updating(), so pages.rds is
   regenerated when settings change.

Verified locally: after applying the fix, custom next labels appear correctly on auto-generated navigation buttons.