feat: Nextcloud 33 compatibility (NC 31–33, PHP 8.2–8.5)

[Copilot]

Brings the app into compliance with Nextcloud 33 requirements: drops PHP 8.1 (removed in NC33), adds PHP 8.5, bumps supported NC range to 31–33, and eliminates all internal OC\* API usage.

Breaking fixes (PHP 8.2+ / NC33)

• FILTER_SANITIZE_STRING removed — replaced with direct casts in AdminSettingsController; added explicit whitelist validation in getCustomization() so unknown provider names return HTTP 400 instead of silently falling through
• OC\Security\CSP\ContentSecurityPolicy → OCP\AppFramework\Http\ContentSecurityPolicy
• OC_Defaults → OCP\Defaults in PersonalSettings and SettingsService
• use OC\AppFramework\Http\Request — removed from four provider files where it was imported but never used

HTTP client modernisation

Replaced all raw curl_* and file_get_contents() calls with Nextcloud's IClientService (respects proxy config, SSL settings, and the new NC33 user-agent). IClientService is injected into the Provider base class and propagated through ProviderDefinitions → SettingsService:

// Before
protected function getData($host) {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $host);
    // ...
    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}

// After
protected function getData(string $host): string|false {
    try {
        return $this->clientService->newClient()->get($host)->getBody();
    } catch (\Exception $e) {
        $this->logger->warning('Failed to fetch data from ' . $host . ': ' . $e->getMessage());
        return false;
    }
}

All concrete providers (WikimediaCommons, WikimediaCommonsDaily, WallhavenCC) now call $this->getData() instead of direct curl_init(). BingWallpaperDaily switches from file_get_contents() to the same.

Dead code removal

• NC ≤28 core.login.showLoginForm route case removed from BeforeTemplateRenderedEventListener (NC ≥29 lowercase form is the only one needed)
• getNextcloudVersion() helper and the > 26 / > 19 version guards removed — always true on NC 31+
• hasDashboard template conditional removed; dashboard has been present since NC 20
• Dead LegacyInitialisationService import removed from Application.php

Version bump

appinfo/info.xml: 3.1.0 → 3.2.0, NC 26–32 → 31–33, PHP 8.0–8.4 → 8.2–8.5