i18n: localize remaining hard-coded UI strings

[maghuro]

Summary

Completes the translation path for UI strings that still bypassed lng() in tinyfilemanager.php.

What changed

• Localizes remaining hard-coded labels and messages in the copy flow, file details, permissions view, archive actions, editor, upload-from-URL flow, backup endpoint, bulk actions, and accessibility labels.
• Adds English fallback keys for every newly localized string.
• Replaces gender-sensitive message assembly with complete translation templates, so languages can express messages such as file/folder creation and deletion naturally.
• Corrects the English confirmation text from Are you sure want to to Are you sure you want to.
• Uses plural labels for file/folder totals.

Scope

This pull request changes only tinyfilemanager.php.

It deliberately does not include any language-file changes; missing translations continue to fall back to English until individual language PRs add them.

Validation

• php -l tinyfilemanager.php passes.
• Tested through PHP's built-in server with a pt-PT configuration: file view, copy view, creation and deletion messages, and localized path labels render successfully.