fix(sidebar): fixed per-Project limits, all Projects, stable groups

[OldKrab]

Problem

Fitting history rows to the viewport (#561) had two defects:

• With many Projects, Projects without live Tasks showed only a header and Load more.
• Collapsing or expanding one Project resized every other Project.

Change

• Removes viewport fitting (useTaskNavigationFit).
• Each Project shows its live rows plus a fixed number of history rows that depends only on the Project count: 20 for one Project, 7 for two, and 3 for three or more.
• Live rows still never count toward the limit, and a collapsed Project still keeps them visible.
• Every Project group renders: the five-Project limit and its "Show N more projects" button are removed.
• An idle open Task keeps its ordinary history slot, so opening a visible Task no longer pulls in an extra row. A Task opened from outside the window, such as from search, is still added.
• docs/task-chat-flow.md records these rules.

Verification

• Frontend: 1514 tests pass, typecheck clean, private-data check passes.
• New tests cover live rows plus three history rows per Project, other Projects keeping their rows when one collapses, all Projects rendering, and a group keeping its size when an idle visible Task opens.
• Browser check at 1280×720 with three Projects: collapsing the first Project leaves the others' rows unchanged.

Known limitation

At 1280×720 with three Projects, when the first has three live rows, the third Project's running Task lands at the bottom edge of the list. A follow-up could add an off-screen "Running below" cue.