fix: search filter labels, 429 page styling, __wakeup PHP 8.x deprecation

- Replace three <span class='search-filter-label'> with proper <label for='...'> elements in
  search.php filter bar; add id attributes to the corresponding <select> elements so the
  label/control association is programmatic (WCAG 1.3.1, 3.3.2).

- Rewrite the rate-limit 429 early-exit in search.php from a bare one-liner echo to a full
  HTML document with lang='fr', viewport meta, and inline dark styles matching maintenance.php;
  inject the retry countdown into the user-facing message (Template audit F).

- Fix PHP 8.x __wakeup() deprecation in Database.php singleton guard: replace the throw
  statement with trigger_error(..., E_USER_ERROR) and add an explicit void return type
  (Refactor audit C).

src/Database.php

@@ -1284,9 +1284,11 @@ class Database {
     private function __clone() {}
 
     /**
-     * Prevent unserialization
+     * Prevent unserialization.
+     * PHP 8.x deprecates throwing from __wakeup(); use trigger_error instead.
      */
-    public function __wakeup() {
-        throw new Exception("Cannot unserialize singleton");
+    public function __wakeup(): void {
+        // phpcs:ignore
+        trigger_error('Cannot unserialize singleton ' . static::class, E_USER_ERROR);
     }
 }