style: retheme system page to site light palette — remove dark --sys-* tokens

TODO.md

@@ -2,4 +2,5 @@
 
 ## Fixes
 - [x] Replace `mb_strlen`/`mb_substr` with `strlen`/`substr` in TfeController, SearchController, Parsedown — mbstring extension not available on production server
+- [x] system.css: retheme to site light palette — remove all `--sys-bg/border/text` dark tokens from variables.css; replace with standard `--bg-*`, `--border-*`, `--text-*` tokens; remap `--sys-syntax-*` to light-appropriate colours; remove dark terminal aesthetic
 - [x] SystemController: PHP-FPM status check — dynamically probe `phpX.Y-fpm` unit derived from running PHP version before static fallback list; add `php8.4-fpm` to cover current production PHP 8.4