improve subtitles for events lasting multiple days

<details>
<summary>Screenshot</summary>

![image](https://github.com/user-attachments/assets/cf1299f6-1d75-43b4-b346-5c000d698de0)

</details>

Dates could be `starts at 13:00`, `all day`, `until 05:00` for example.

Files to modify :
- `src/utils/Planning.ts`
- `__tests__/utils/PlanningEventManager.test.js`