project_panel: Fix divider taking too much horizontal space (#44920)

Closes: #44917

While setting up the project for contribution, I noticed that the
divider in the welcome dialog was rendering incorrectly on the `main`
branch compared to the latest release.

**Current behaviour (`main` branch):**
<img width="796" height="690" alt="image"
src="https://github.com/user-attachments/assets/3f7d6c73-14eb-47f3-ad83-4796f5f7be0f"
/>

**Expected behaviour (Release `0.216.1`):**
<img width="794" height="692" alt="image"
src="https://github.com/user-attachments/assets/b67857dc-a03d-4e49-bb33-22fe0c83ac5d"
/>

---

After some investigation, it looks like the issue was introduced in
#44505, specifically in [these
changes](https://github.com/zed-industries/zed/pull/44505/changes#diff-4ea61133da5775f0d5d06e67a8dccc84e671c3d04db5f738f6ebfab3a4df0b01R147-R158),
which caused the divider to take the full width instead of being
properly constrained.

**PR result**:
<img width="666" height="574" alt="image"
src="https://github.com/user-attachments/assets/e12b7778-b7cc-4855-b82e-3470dfe43365"
/>

Release Notes:

- Fixes -or- divider rendering incorrectly

crates/project_panel/src/project_panel.rs

@@ -6050,9 +6050,9 @@ impl Render for ProjectPanel {
                     h_flex()
                         .w_1_2()
                         .gap_2()
-                        .child(Divider::horizontal())
+                        .child(div().flex_1().child(Divider::horizontal()))
                         .child(Label::new("or").size(LabelSize::XSmall).color(Color::Muted))
-                        .child(Divider::horizontal()),
+                        .child(div().flex_1().child(Divider::horizontal())),
                 )
                 .child(
                     Button::new("clone_repo", "Clone Repository")