system.php: extract inline JS and style= attrs into separate assets

Move the ~130-line $extraJsInline heredoc from admin/system.php into a
static file public/assets/js/system.js, loaded via $extraJs so the
template footer emits a normal <script src=…>.

Replace 4 inline style= attributes with named CSS modifier classes in
system.css:
  - style="margin:0;border:none;padding:0" on .srv-section-title
    → .srv-section-title--compact
  - style="margin-bottom:.75rem" on sub-heading <h3>
    → .srv-section-title--sub
  - style="margin-bottom:0" on .php-grid
    → .php-grid--flush
  - style="font-size:.84rem;color:var(--text-secondary)" on <label>
    → .log-toolbar label rule in system.css

The one remaining inline style (--disk-pct / --disk-color CSS custom
properties on .disk-bar) is intentionally kept: it carries PHP runtime
values that cannot be expressed in a static stylesheet.

todo/02-php-components.md

@@ -23,7 +23,7 @@
 - [ ] Consolidate action handlers into controller methods
 - [x] Unify flash message keys project-wide to `_flash_error` / `_flash_success` — all callers already use `App::flash()`; removed dead legacy-key fallback chains (`error`, `admin_error`, `edit_error`, `form_error`, `success`, `admin_success`, `edit_success`) from `consumeFlash()`
 - [ ] Move OG tag construction into controller logic
-- [ ] Extract inline CSS/JS from `system.php` into separate assets
+- [x] Extract inline CSS/JS from `system.php` into separate assets — JS moved to `public/assets/js/system.js` (loaded via `$extraJs`); 4 inline `style=` attributes replaced with CSS classes; only dynamic CSS custom properties (`--disk-pct`, `--disk-color`) remain as inline styles because they carry PHP runtime values
 
 ## Backend Maintenance