fix: clicking a sidebar section header did nothing

The sidebar has two activation paths and they had drifted apart.
handleEnter dispatches on three row kinds -- the Threads row, a section
header (toggle collapse), and a channel -- while reduceMouseClick
handled only two of them, channels and Threads.

ClickAt returns ok=false for every non-channel row, and the mouse
reducer treated that as "nothing here" after a single special case for
the Threads row. So clicking a section header moved the sidebar
selection and drew the highlight, then returned nil. The header looked
live and did nothing; collapsing a section was keyboard-only.

This is most visible on the Channels section, which ships collapsed by
default -- a new user's first instinct is to click it open.

Adds the missing branch, in handleEnter's order so the two paths read
as the same decision.

The test sweeps every rendered row and asserts click and Enter agree on
each, rather than asserting one row kind at a time. That is deliberate:
the row that broke here is precisely the row nobody thought to write a
case for, so a per-kind test would have had the same blind spot. It
fails on the defect at the section-header row and would fail again for
the next row kind wired into one path and not the other.

Verified by reverting the fix and confirming the test fails on it.