[desktop] Add mod importer from folder and zip (#3472)

Closes #3125

Adds buttons to the addons page that imports a mod (or mods) from zip or folder.

Currently known to work with mods that provide proper romfs/exefs things, unsure about cheats and such. Also works on mods that just stuff things into the root of the zip.

TODO:
- [ ] test folder more thoroughly
- [ ] cheats
- [ ] test all sorts of mod pack types

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3472
Reviewed-by: Lizzie <lizzie@eden-emu.dev>

src/frontend_common/mod_manager.h

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <filesystem>
+#include <optional>
+#include "common/common_types.h"
+
+namespace FrontendCommon {
+
+enum ModInstallResult {
+    Cancelled,
+    Failed,
+    Success,
+};
+
+std::vector<std::filesystem::path> GetModFolder(const std::string& root);
+
+ModInstallResult InstallMod(const std::filesystem::path &path, const u64 program_id, const bool copy = true);
+}