Internationalise user-facing strings

[GaryJones]

Summary

Most of the plugin's output was already wrapped in translation functions, but a handful of user-facing strings had been left as hard-coded English: the four front-end flagging messages (thank you, invalid nonce, invalid values, already flagged) and the default "Report comment" link text.

None of these could simply be wrapped in __() where they lived. The messages were property defaults and the link text was a method parameter default, and PHP forbids function calls in both. Assigning the messages in the constructor was not an option either, because the plugin is instantiated at load time, before init, and calling __() that early triggers WordPress 6.7's "translation loading triggered too early" notice.

To resolve this, the translatable defaults are now assigned on init via a new set_default_messages() method, which also runs the existing safe_report_comments_* message filters. As a natural consequence those filters now fire on init rather than at construction, which is strictly an improvement: filters added on plugins_loaded or init previously ran too late to take effect. The link text is translated lazily inside get_flagging_link(), which only ever runs well after init.

Test plan

• With a translation loaded for the safe-report-comments text domain, confirm the report link text and the flagging response messages appear translated.
• Confirm no "translation loading triggered too early" notice appears with WP_DEBUG enabled on WordPress 6.7+.
• Confirm the safe_report_comments_*_message and safe_report_comments_flagging_link_text filters still override the defaults.