[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.cpp

@@ -0,0 +1,62 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include <algorithm>
+#include <filesystem>
+#include <fmt/format.h>
+#include "common/fs/fs.h"
+#include "common/fs/fs_types.h"
+#include "common/logging/backend.h"
+#include "frontend_common/data_manager.h"
+#include "mod_manager.h"
+
+namespace FrontendCommon {
+
+// TODO: Handle cases where the folder appears to contain multiple mods.
+std::vector<std::filesystem::path> GetModFolder(const std::string& root) {
+    std::vector<std::filesystem::path> paths;
+
+    auto callback = [&paths](const std::filesystem::directory_entry& entry) -> bool {
+        const auto name = entry.path().filename().string();
+        static constexpr const std::array<std::string, 5> valid_names = {"exefs",
+                                                                         "romfs"
+                                                                         "romfs_ext",
+                                                                         "cheats", "romfslite"};
+
+        if (std::ranges::find(valid_names, name) != valid_names.end()) {
+            paths.emplace_back(entry.path().parent_path());
+        }
+
+        return true;
+    };
+
+    Common::FS::IterateDirEntriesRecursively(root, callback, Common::FS::DirEntryFilter::Directory);
+
+    return paths;
+}
+
+ModInstallResult InstallMod(const std::filesystem::path& path, const u64 program_id, const bool copy) {
+    const auto program_id_string = fmt::format("{:016X}", program_id);
+    const auto mod_name = path.filename();
+    const auto mod_dir =
+        DataManager::GetDataDir(DataManager::DataDir::Mods) / program_id_string / mod_name;
+
+    // pre-emptively remove any existing mod here
+    std::filesystem::remove_all(mod_dir);
+
+    // now copy
+    try {
+        std::filesystem::copy(path, mod_dir, std::filesystem::copy_options::recursive);
+        if (!copy)
+            std::filesystem::remove_all(path);
+    } catch (std::exception& e) {
+        LOG_ERROR(Frontend, "Mod install failed with message {}", e.what());
+        return Failed;
+    }
+
+    LOG_INFO(Frontend, "Copied mod from {} to {}", path.string(), mod_dir.string());
+
+    return Success;
+}
+
+} // namespace FrontendCommon