Fix deprecated zip functions and undefined variable warnings in problem import

Details:
- Replace deprecated `zip_open()`, `zip_read()`, `zip_entry_*` functions with `ZipArchive` class in `problem_import_syzoj.php` and `problem_import_hoj.php`.
- Fix "Undefined variable $spjlang" warning in `problem_import_xml.php` by adding `isset()` check.
- Remove usage of undefined `$path` variable in import scripts.

Fix: Resolve PHP 8 deprecations, undefined warnings, and import logic bugs

1. Fix Problem Import Logic (problem_import_xml.php):
   - Fix "Undefined variable $spjlang" warning.
   - Fix bug where empty `remote_oj` tags in XML caused existing problems to be overwritten instead of creating new ones.
   - Add `unset($spjlang)` and `unset($tpjlang)` in batch import loop to prevent SPJ attributes from leaking to subsequent problems.
   - Replace deprecated `zip_open`/`zip_read` functions with `ZipArchive` class.

2. Fix Zip Import Handlers (problem_import_syzoj.php, problem_import_hoj.php):
   - Replace deprecated `zip_*` functions with `ZipArchive` for PHP 8 compatibility.
   - Remove redundant double-loop processing in HOJ import.
   - Add directory existence checks.

3. Fix Admin Warnings (problem_df_change.php, contest_add.php, news_add_page.php):
   - Add `isset($_POST['hlist'])` checks to resolve "Undefined array key" and "trim(): Passing null" warnings.