Introduce FR_EXEC_PROBE config constant (WorkerLauncher + controllers)

[Giermann]

Add a new config constant FR_EXEC_PROBE (default: true). When set to false, FileRise treats exec/shell_exec as completely unavailable, regardless of what isFunctionEnabled() reports.

Changes to WorkerLauncher:

• resolvePhpCli(): return null immediately when FR_EXEC_PROBE === false, before any filesystem checks
• canSpawnBackground(): add FR_EXEC_PROBE === false guard; also call resolvePhpCli() to verify a working binary actually exists
• canRunForeground(): add prefersSync() short-circuit (no exec needed in sync mode); add FR_EXEC_PROBE === false guard; call resolvePhpCli() to verify
• hasShell(): add @ operator to is_file('/bin/sh') and is_executable('/bin/sh') to suppress open_basedir warnings before they reach the custom error handler

Changes to FileController and FolderController:

• Add runJobInRequest() private method that executes file/folder move and copy operations synchronously in-request via FileModel directly (for files) and via FolderController::moveFolder() with output buffering (for folders). Sets the job file to "done" or "error" on completion.
• In enqueueTransferJob(): add an early guard before spawnWorker() - if !canSpawnBackground() && !canRunForeground(), call runJobInRequest() directly instead of spawning a worker.
• Also call runJobInRequest() as a second fallback after a failed spawn attempt, so that unexpected spawn failures on other restricted hosts are handled gracefully too.
• In FileController::transferJobStart(): respect FR_WORKER_MODE=sync by setting $payload['async'] = !WorkerLauncher::prefersSync(). When sync mode is active, capture the response via ob_start()/ob_get_clean() and wrap it in a job-compatible envelope (ok, jobId, status: done) so the frontend poll loop receives the expected structure.
• In FileController::downloadZip(): add an early guard - if no exec is available and format is zip, fall back to synchronous FileModel::createZipArchive() (pure PHP, no exec needed). For 7z format, return a clear 501 error immediately instead of timing out.
• In FileController::thumbnail(): replace !function_exists('exec') with !WorkerLauncher::canRunForeground() and return HTTP 204 (no content) instead of 501, so the browser silently skips the thumbnail without showing an error to the user.

To use on a restricted shared host, add to config.php:

define('FR_WORKER_MODE', 'sync');
define('FR_EXEC_PROBE', false);