fix(webui): make the backups filter placeholder readable

The snapshot filter's placeholder had no rule at all, so it fell through
to the browser default — a grey chosen for light backgrounds, rendering
near-invisible against a dark panel. "Filter by app, host, or backup id"
is the one hint telling you what the box accepts, so losing it costs the
control its explanation.

Written against --text-rgb rather than a fixed grey so it follows the
theme: white at 60% on nebula and dark-blue, dark at 60% on light, where
a hardcoded #ccc would have been unreadable the other way round. Firefox
dims placeholders on top of the colour, so opacity is pinned to 1 and the
alpha above owns the result. Slightly brighter than the task filter's
0.4, and in line with the shared .form-control rule's effective value,
because this string is instruction rather than decoration.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

containers/libreportal/frontend/components/backup/core/css/backup.css

@@ -641,6 +641,19 @@
     min-width: 0;
 }
 
+/* The placeholder had no rule at all, so it fell through to the browser
+   default — which on a dark theme renders near-invisible grey against the
+   panel, and "Filter by app, host, or backup id" is the one hint telling you
+   what the box accepts. Written against --text-rgb rather than a fixed grey so
+   it follows the theme: white at 60% on nebula/dark-blue, dark at 60% on light,
+   where a hardcoded #ccc would be unreadable the other way round. Slightly
+   brighter than the task filter's 0.4 because this string is instruction, not
+   decoration. */
+.backup-filter-input::placeholder {
+    color: rgba(var(--text-rgb), 0.6);
+    opacity: 1; /* Firefox dims placeholders by default; the alpha above owns it */
+}
+
 .backup-filter-input:focus,
 .backup-filter-select:focus {
     outline: none;