R8 phase 1: extract app/engine shared utilities into shared/include/oakutil

Move the pure-header utilities shared by app/ and engine/ out of
engine/common/ into a new shared/include/oakutil/ layer (define, lerp,
decibel, digit, range, crashpadutils, autoscroll, qtutils, filefunctions
declarations, and a trimmed xmlutils exposing a CancelAtom-free void*
overload). engine/common/ keeps forwarding headers so internal include
paths are unchanged; app/ now includes oakutil/* directly.

engine/node/project.h gains an explicit NodeGroup forward declaration
previously obtained transitively through the old xmlutils.h.

engine/common/filefunctions.h

@@ -1,109 +1,2 @@
-/***
-
-  Olive - Non-Linear Video Editor
-  Copyright (C) 2022 Olive Team
-  Modifications Copyright (C) 2025 mikesolar
-
-  This program is free software: you can redistribute it and/or modify
-  it under the terms of the GNU General Public License as published by
-  the Free Software Foundation, either version 3 of the License, or
-  (at your option) any later version.
-
-  This program is distributed in the hope that it will be useful,
-  but WITHOUT ANY WARRANTY; without even the implied warranty of
-  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-  GNU General Public License for more details.
-
-  You should have received a copy of the GNU General Public License
-  along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-***/
-
-#ifndef OAK_FILEFUNCTIONS_H
-#define OAK_FILEFUNCTIONS_H
-
-#include <QDir>
-#include <QString>
-
-#include "common/define.h"
-
-namespace olive
-{
-
-/**
- * @brief A collection of static file and directory functions
- */
-class FileFunctions {
-public:
-	/**
-   * @brief Returns true if the application is running in portable mode
-   *
-   * In portable mode, any persistent configuration files should be made in a path relative to the application rather
-   * than in the user's home folder.
-   */
-	static bool is_portable();
-
-	static QString get_unique_file_identifier(const QString &filename);
-
-	static QString get_configuration_location();
-
-	static QString get_application_path();
-
-	static QString get_temp_file_path();
-
-	static bool can_copy_directory_without_overwriting(const QString &source,
-												   const QString &dest);
-
-	static void copy_directory(const QString &source, const QString &dest,
-							  bool overwrite = false);
-
-	static bool directory_is_valid(const QDir &dir,
-								 bool try_to_create_if_not_exists = true);
-
-	/**
-   * @brief Ensures a given filename has a certain extension
-   *
-   * Checks if the filename has the extension provided and appends it if not. The extension is
-   * checked case-insensitive. The extension should be provided with no dot (e.g. "ove" rather than
-   * ".ove").
-
-   * @return The filename provided either untouched or with the extension appended to it.
-   */
-	static QString ensure_filename_extension(QString fn,
-										   const QString &extension);
-
-	static QString read_file_as_string(const QString &filename);
-
-	/**
-   * @brief Returns a temporary filename that can be used while writing rather than the original
-   *
-   * If overwriting a file, it's safest to write to a new file first and then only replace it at
-   * the end so that if the program crashes or the user cancels the save half way through, the
-   * original file is still intact.
-   *
-   * This function returns a slight variant of the filename provided that's guaranteed to not exist
-   * and therefore won't overwrite anything important.
-   */
-	static QString get_safe_temporary_filename(const QString &original);
-
-	/**
-   * @brief Renames a file from `from` to `to`, deleting `to` if such a file already exists first
-   */
-	static bool rename_file_allow_overwrite(const QString &from,
-										 const QString &to);
-
-	inline static QString get_formatted_executable_for_platform(QString unformatted)
-	{
-#ifdef Q_OS_WINDOWS
-		unformatted.append(QStringLiteral(".exe"));
-#endif
-
-		return unformatted;
-	}
-
-	static QString get_auto_recovery_root();
-};
-
-}
-
-#endif // OAK_FILEFUNCTIONS_H
+// Forwarding header - canonical content moved to shared/include/oakutil/filefunctions.h
+#include "oakutil/filefunctions.h"