#34509 Agent chat: a long name gives way after the preview, so the chevron stays in view

Follow-up to #34464, from its review. Leo's ask there:

idk why names are ellipses'd so early plus the weird chevron spacing thing

#34464 took the 40%/45% caps off names and stopped them shrinking. That left a gap: in a narrow pane, a long name could push the chevron and the status off the row.

What changes

• A name still keeps its natural width while the message's first line has room to give.
• Once the first line is gone, the name is cut instead of the chevron or status. It does this with a flex-shrink of 0.0001: below a layout unit while the preview still has room, and all of the remaining overflow once the preview is gone.
• A received line's toggle never shrinks below the chevron's own room.
• A send's file name is cut with an ellipsis, the same way the preview is.
• The declined message line's chevron now uses the rows' 14/1.2, like every other row chevron.
• Left as they were: the Details and Raw toggles inside an opened card. They're in-card controls, not row disclosures, so they keep 12/1.4.

This is a restyle only. Nothing is removed or switched off.

Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Fixes long agent names pushing the chevron and status off the message row in narrow panes.

• Names keep their natural width while the preview gives way first; once the preview is gone, the name is cut instead of the chevron or status.
• A received line's toggle gives way before its sender's name, shrinking down to the chevron's own room but never under it.
• A send's file name is ellipsized like the preview.
• The declined line's chevron now matches the rows' 14/1.2.