Fix cache directory permissions and PHP 8+ TypeError on fopen failure

[toineenzo]

Summary

Two small, backwards-compatible fixes in plugin.php:

1. mkdir() mode 0600 -> 0755

On activation, the plugin creates a cache/ directory with mode 0600. That mode (rw- for owner only, no execute bit) prevents PHP from traversing the directory it just created, so subsequent file_exists(), filemtime(), and fopen() calls against files inside cache/ silently fail. Switching to 0755 makes the directory actually usable.

2. Guard fwrite()/fclose() against fopen() failure

ozh_ycache_shutdown() did:

$fp = fopen( $cachefile, 'w' );
$data = ob_get_contents();
fwrite($fp, $data);
fclose($fp);

If fopen() fails (cache dir missing, unwritable, full disk, etc.), it returns false. Since PHP 8.0, fwrite() and fclose() require a resource and throw TypeError when given false, which would crash the request. Added an explicit if ( $fp !== false ) check around the writes. The buffered output is still echoed to the browser either way, so a write failure now degrades gracefully instead of fatally.

No version bump, no behavior change on the happy path.

Test plan

• Activate plugin on a fresh install; confirm cache/ is created and writable by PHP
• Visit a stat page (+) twice, confirm second hit serves from cache (<!-- Cached ... --> marker)
• Manually chmod 000 cache/ and visit a stat page; confirm no fatal TypeError, page still renders

[ozh]

Thanks. I think this PR fixes two problems that don't exist :

• Minor : changing fopen() to @fopen is surely an edge case we might want to deal with, but honestly who in their right mind would activate a plugin then manually delete the folder.
• Major : as is, the plugin works fine -- see last line of view-source:https://app.yourls.org/protonpass+ for instance -- so I don't get in what case it would silently fail. And chmoding to 0755 on shared hosting is a super bad idea : any user can see your dir content. If anything, 0700 would be a slightly better value, if there is something to fix in the first place (ie a case where 0600 would actually fail)