bug(workspace): the agent band's activity tooltip opens upward and is cut off under the header #3264

What happens

On the Workspace agent band, hovering a bar in the Activity · last 7 days strip opens the breakdown tooltip upward, and its top rows slide under the page header. In the reported screenshot (agent band showing 99% first try, hovered day: Scheduled 11 · Reminders 6 · Agent-to-agent 4 · Total 25), the "Tool call" row is cut in half by the header and anything above it is hidden. The day's date and the top buckets can't be read.

Why

src/frontend/src/components/StackedBarChart.vue positions the tooltip with absolute bottom-full … z-20, so it always opens above the bar. In PortalAgentBand.vue the chart is 23px tall (:height="23", legend="none", :axis="false") and sits just under the header, so there is never room above it. With the legend and axis removed on purpose, the tooltip is the only place that names the buckets and the date, so clipping it hides that information entirely.

Expected

The whole tooltip is visible wherever the chart sits. It either flips below the bar when there is no room above, or renders outside the clipping/stacking context (e.g. teleported) so the header cannot cover it.

Acceptance criteria

• On the Workspace agent band, hovering any bar shows the complete tooltip (date, every bucket, total) with no part covered by the header or cut by a parent.
• Same check on the other StackedBarChart uses (agent Overview panel), so they don't regress.
• Bars near the left and right edges keep the tooltip inside the viewport horizontally as well.

Reported from Workspace dogfooding (screenshot held by the reporter; it can be dropped into this issue in the browser).