lut: pick from the global LUT library in node params; i18n; doc

- File inputs marked with the 'lut_library' property (currently the
  OCIO LUT node) now show a combo box populated from the global LUT
  library above the path field: pick a library LUT directly, or use
  'Other (Custom File)...' with the regular path field; selections go
  through the standard undoable input-change path
- Refresh zh_CN translations with lupdate and translate all strings
  introduced by the proxy dialog, LUT library, LUT picker, waveform
  sync and footage start time work
- Document the new per-footage custom <proxy> attributes and the
  'manual' source-start-time origin in the project file reference
- Add LutFileField UI tests

app/widget/filefield/CMakeLists.txt

@@ -18,5 +18,7 @@ set(OLIVE_SOURCES
         ${OLIVE_SOURCES}
         widget/filefield/filefield.cpp
         widget/filefield/filefield.h
+        widget/filefield/lutfilefield.cpp
+        widget/filefield/lutfilefield.h
         PARENT_SCOPE
 )

app/widget/filefield/filefield.h

@@ -38,7 +38,7 @@ public:
 		return line_edit_->text();
 	}
 
-	void SetFilename(const QString &s)
+	virtual void SetFilename(const QString &s)
 	{
 		line_edit_->setText(s);
 	}

app/widget/filefield/lutfilefield.cpp

@@ -0,0 +1,89 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  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/>.
+
+***/
+
+#include "lutfilefield.h"
+
+#include <QDir>
+#include <QHBoxLayout>
+
+#include "render/lutlibrary.h"
+
+namespace olive
+{
+
+LutFileField::LutFileField(QWidget *parent) : FileField(parent)
+{
+	library_combo_ = new QComboBox();
+	library_combo_->setSizeAdjustPolicy(QComboBox::AdjustToMinimumContentsLengthWithIcon);
+	library_combo_->setMinimumContentsLength(12);
+	static_cast<QHBoxLayout *>(layout())->insertWidget(0, library_combo_, 1);
+
+	RefreshLibraryEntries();
+
+	connect(library_combo_,
+			static_cast<void (QComboBox::*)(int)>(&QComboBox::activated), this,
+			[this](int index) {
+				const QString path =
+					library_combo_->itemData(index).toString();
+				if (!path.isEmpty()) {
+					SetFilename(path);
+					emit FilenameChanged(path);
+				}
+			});
+
+	// Keep the combo in sync when the path is edited directly
+	connect(this, &FileField::FilenameChanged, this, [this](const QString &) {
+		RefreshLibraryEntries();
+	});
+}
+
+void LutFileField::SetFilename(const QString &s)
+{
+	FileField::SetFilename(s);
+	RefreshLibraryEntries();
+}
+
+void LutFileField::RefreshLibraryEntries()
+{
+	const QString current = GetFilename();
+
+	const QSignalBlocker blocker(library_combo_);
+	library_combo_->clear();
+	library_combo_->addItem(tr("Other (Custom File)..."), QString());
+
+	const QStringList library_dirs = LUTLibrary::GetDirectories();
+	const QStringList luts = LUTLibrary::GetLutFiles();
+	for (const QString &lut : luts) {
+		// Show the path relative to the library directory that contains it
+		QString display = lut;
+		for (const QString &dir : library_dirs) {
+			if (lut.startsWith(dir)) {
+				display = QDir(dir).relativeFilePath(lut);
+				break;
+			}
+		}
+		library_combo_->addItem(display, lut);
+	}
+
+	const int index = library_combo_->findData(current);
+	library_combo_->setCurrentIndex(index >= 0 ? index : 0);
+}
+
+}

app/widget/filefield/lutfilefield.h

@@ -0,0 +1,71 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  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 LUTFILEFIELD_H
+#define LUTFILEFIELD_H
+
+#include <QComboBox>
+
+#include "filefield.h"
+
+namespace olive
+{
+
+/**
+ * @brief A FileField with a combo box for picking LUTs from the global LUT
+ * library
+ *
+ * The combo lists every LUT found by LUTLibrary::GetLutFiles() plus an
+ * "Other" entry for custom file paths. Picking a library entry fills in the
+ * file path (through the regular FilenameChanged signal, so undo keeps
+ * working); picking "Other" or entering a path that is not in the library
+ * leaves the path untouched and shows the combo's "Other" entry.
+ */
+class LutFileField : public FileField {
+	Q_OBJECT
+public:
+	LutFileField(QWidget *parent = nullptr);
+
+	virtual void SetFilename(const QString &s) override;
+
+	/**
+	 * @brief The combo box listing the LUT library entries
+	 *
+	 * Exposed for inspection and UI tests; prefer SetFilename()/GetFilename()
+	 * for interacting with the field itself.
+	 */
+	QComboBox *library_combo() const
+	{
+		return library_combo_;
+	}
+
+private:
+	/**
+	 * @brief Repopulates the combo from the LUT library and syncs the
+	 * selection with the current filename
+	 */
+	void RefreshLibraryEntries();
+
+	QComboBox *library_combo_;
+};
+
+}
+
+#endif // LUTFILEFIELD_H

app/widget/nodeparamview/nodeparamviewwidgetbridge.cpp

@@ -42,6 +42,7 @@
 #include "widget/bezier/bezierwidget.h"
 #include "widget/colorbutton/colorbutton.h"
 #include "widget/filefield/filefield.h"
+#include "widget/filefield/lutfilefield.h"
 #include "widget/slider/floatslider.h"
 #include "widget/slider/integerslider.h"
 #include "widget/slider/rationalslider.h"
@@ -141,7 +142,15 @@ void NodeParamViewWidgetBridge::CreateWidgets()
 			break;
 		}
 		case NodeValue::kFile: {
-			FileField *file_field = new FileField(parent);
+			FileField *file_field;
+			if (GetInnerInput().GetProperty(QStringLiteral("lut_library"))
+					.toBool()) {
+				// File inputs that accept LUTs get a combo box for picking
+				// from the global LUT library
+				file_field = new LutFileField(parent);
+			} else {
+				file_field = new FileField(parent);
+			}
 			widgets_.append(file_field);
 			connect(file_field, &FileField::FilenameChanged, this,
 					&NodeParamViewWidgetBridge::WidgetCallback);