Extract last 3 inline styles from admin templates into CSS classes

admin/thanks.php:
- <div style="margin-top:1.5rem;display:flex;gap:.75rem;flex-wrap:wrap;"> → class="admin-action-bar"
- <p style="color:var(--text-secondary);"> → class="admin-muted"

admin/pages.php:
- Éditer button style="font-size:.8rem;padding:.3rem .75rem;" → class="admin-btn admin-btn--sm"

admin.css (Thesis info sections block):
- Added .admin-action-bar { margin-top:1.5rem; display:flex; gap:0.75rem; flex-wrap:wrap }
- Added .admin-muted { color: var(--text-secondary) }

The only remaining inline style in any admin PHP file is the dynamic
--disk-pct/--disk-color custom properties on the disk bar in system.php,
which carry PHP runtime values and cannot be moved to static CSS.

TODO.md

@@ -11,6 +11,8 @@ Pending tasks have been split into topic files under [`todo/`](todo/README.md):
 
 ## Recently completed (this session)
 
+- [x] `admin/thanks.php` + `admin/pages.php` + `admin.css` — extracted last 3 inline `style=` attributes from admin PHP templates: `<div style="margin-top:1.5rem;display:flex;gap:.75rem;flex-wrap:wrap;">` in `thanks.php` → `class="admin-action-bar"` (`.admin-action-bar` rule added to CSS); `<p style="color:var(--text-secondary);">` in `thanks.php` → `class="admin-muted"` (`.admin-muted` rule added); `style="font-size:.8rem;padding:.3rem .75rem;"` on Éditer button in `pages.php` → `class="admin-btn admin-btn--sm"` (uses existing modifier). Only the dynamic `--disk-pct`/`--disk-color` CSS custom properties on the disk bar in `system.php` remain (carry PHP runtime values — legitimate).
+
 - [x] `variables.css` + `search.css` — public dark-mode support via `@media (prefers-color-scheme: dark)` scoped to `body:not(.admin-body)`: all semantic tokens (`--bg-*`, `--text-*`, `--border-*`, `--accent-*`, status colours) overridden with dark equivalents; `--accent-primary` lightened to `#b87fd4` for WCAG contrast on dark backgrounds; `--search-error-*` variables added to replace hardcoded `#fff0f0`/`#c00` in `.search-error`; admin pages unaffected