When a subagent title is long, the header buttons (expand/collapse,
maximize, stop) were pushed out of view because the title area grew
unconstrained.
## Fix
- Add `min_w_0()` and `overflow_hidden()` to the title container so it
can shrink below its content width
- Add `truncate()` to the title label so it shows an ellipsis when
clipped
- Add `flex_shrink_0()` to the buttons container so it never gets
squeezed out
- Add a tooltip on the title area showing the full text on hover
Release Notes:
- N/A
Eric Holk
41c007f9ee