For the longest time, we've had a labeled "Share" button on the title
bar whenever you're in a call. However, it was extremely easy to click
that button thinking you'd be sharing _your screen_ instead of your
project. At the end of the day, there wasn't any specific cue that'd
allow you to know what you'd be sharing. This is why people just assumed
you'd share the screen, which is arguably way more common than sharing
the project. We've seen users bump into this confusion countless times,
and this also even happens to us (the whole Zed staff) weekly whenever
we're in meetings.

So, this PR changes that button to be an icon button instead, using a
folder icon to better communicate it means sharing the project. I also
took advantage of the opportunity to change the approach we use for
adding these buttons, using `when` instead of `children.push`; I think
the former is just easier to understand, allowing to more easily picture
the UI in your (my) head. Lastly, also improved what we display in the
tooltip for the project share button, by showing the names of all
projects you'll be sharing, as well as the tooltip that displayed call
diagnostics.

| Before | After |
|--------|--------|
| <img width="778" height="360" alt="Screenshot 2026-06-05 at 3  48
2@2x"
src="https://github.com/user-attachments/assets/363593fb-79f8-42e9-83b9-345bc731846f"
/> | <img width="778" height="360" alt="Screenshot 2026-06-05 at 3  48
3@2x"
src="https://github.com/user-attachments/assets/32afc412-a1fb-45a4-a19e-74e4923c0c8b"
/> |

Release Notes:

- Improved call controls design, making it clearer what button shares
_the project_ vs. _the screen_.