feat: refactor activity window into own folder structure

[Rello]

Implemented the Search-style activity modularization.

• Moved activity QML and C++ support into src/gui/activity/ and src/gui/activity/qml/.
• Updated resources, CMake, imports, and external include paths.
• Activities window now receives account data, activity model, and sync-status model from Systray, rather than accessing UserModel from QML.
• Moved chat replies into ActivityListModel, removing the User-model reply bridge.
• Added account-based sync-status loading for the standalone window.

[chatgpt-codex-connector]

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[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot
47.1% Coverage on New Code (required ≥ 80%)
141 New Code Smells (required ≤ 0)

See analysis details on SonarQube Cloud

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[Rello]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Done — merged origin/master into the branch. Git auto-resolved all conflicts cleanly (resources.qrc, CMakeLists.txt, activitylistmodel.cpp, owncloudgui.cpp, systray.cpp). The merge commit is c2a23ed.

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-10350.zip

Digest: sha256:466d49bd02a42aa4f2da394aec6482baaa5e30a62485cab6c873f5144e3867b5

To test this change/fix you can download the above artifact file, unzip it, and run it.

Please make sure to quit your existing Nextcloud app and backup your data.