[Bug]: Sidebar drops the first project's header while its workspace stays, so a workspace renders under the wrong project until you scroll

### What happens

The first project in the sidebar loses its header row while its workspace row stays. Everything below shifts up one slot: the first project's workspace is drawn under the **second** project's header, and the second project's own workspace ends up with no header above it at all. Projects further down are unaffected.

Scrolling the sidebar fixes it immediately, and it stays fixed.

### Frames

**1 — cold launch.** Five project headers, no workspace rows yet.

**2 — after the workspace rows arrive.** A's header is gone; A's workspace sits under B's header; B's workspace has none.

**3 — collapsing B proves the grouping data is correct.** Collapsing project B hides only B's workspace — the orphaned block. A's workspace stays visible, because it is not B's.

**4 — settled.** Each project holds its own workspace.

(Names and session titles are redacted; folder icons are left visible so the header count is checkable.)

### What narrows it down

- **Only the first project is affected.** Projects lower in the list render correctly even with several agent sessions each.
- **The grouping data is intact.** Frame 3: collapsing B hides B's workspace and leaves A's alone, so the rows know which project they belong to. Only the header row is missing.
- **Toggling row heights flips it.** Expanding/collapsing the agent list under the first project's workspace moves it between the broken and correct layouts.
- **Scrolling repairs it.** Shrinking the window until a scrollbar appeared and scrolling restored the correct layout; going back to fullscreen kept it correct.
- **Not a filter.** Every sidebar filter is off — hide sleeping, hide default branch, hide automation-created, hide CLI-created, hide detached HEAD.

Together those say the data is right and something in row layout is not, until a re-measure is forced.

### Repro

1. Import several projects; give the **first** one a workspace with a few agent sessions.
2. Launch Orca cold, group by project, all filters off.
3. Watch the sidebar as the workspace rows fill in — the first project's header disappears and the rows below shift up.
4. Scroll the sidebar. The layout corrects itself.

Seen twice on separate launches, the same project each time.

### Related but not the same

#16247 describes the launch-time list settling and traces it to data arriving late (`undefined` read as `[]`; PTY not yet reattached read as asleep). This looks like a different path: the association is already correct when the frame is wrong, the fix is a scroll rather than more data, only the first row group is hit, and it happens with the hide-sleeping filter off — which that issue's second cause depends on.

### Environment

Windows 11, Orca 1.4.194. Five imported projects, all local — no remote runtime hosts, no paired Orca servers. Group by project, show all projects, sort by agent activity.