💡 Add Option to Edit Username in Proposal Discussion Forum

### Area

Other

### Is there new design needed?

Yes

### What?

Introduce an option to **edit the username** for the **Proposal Discussion Forum**. Currently, users can set their username during their first visit, but there is no way to modify it afterward.

The username should be displayed in the **sidebar at the bottom** as shown in the provided [Figma design](https://www.figma.com/design/mJTBiUofLfb74Xc3W2U59o/branch/JYhRTShAqT5k12vYwQRpuk/Voltaire---GOV-TOOL) and should include an **inline editing** option for easy updates.

### Why?

- Allows users to correct mistakes in their initial username entry.
- Provides flexibility for users to update their username to better reflect their identity.
- Improves user experience by aligning with common profile management features.

### How?

- Display the **username** in the **sidebar at the bottom** as shown in the Figma design.
- Add an **inline editing** feature for seamless username updates.
- Implement appropriate **validation rules** (e.g., character limits, restricted symbols).
- Ensure **real-time feedback** during editing (e.g., availability check for unique usernames).

### (Optional) User Story with acceptance criteria

- [ ] Username is displayed in the **sidebar at the bottom** as per the provided Figma design.
- [ ] Users can **edit their username** via inline editing directly from the sidebar.
- [ ] Changes to the username are updated and displayed correctly across the forum.
- [ ] Appropriate **validation rules** are enforced during editing. /