Polish projects UX

- [ ] Include a last modified time in projects and show this on the title screen
- [x] Save as / make copy of a project?
- [x] Title screen shouldn't have a map at all
- [x] Internally use `ltn_name.geojson`, but don't show the prefix and suffix in the UI
- [ ] Warn when clobbering an existing name for both projects and neighbourhoods
- [x] More granular progress bar
- [ ] Handle multiple proposals in one project?

该 issue 描述了针对项目的几项 UX 改进。首先探索处理项目标题、保存和警告的 frontend 代码。查找显示项目列表和标题屏幕的 UI 组件。检查项目文件在内部是如何命名的，以及它们是如何显示的。实现“最后修改时间”和“覆盖时发出警告”功能需要同时修改 UI 逻辑以及项目保存/加载例程。

由索引模型根据 Issue 内容生成。