fix(charts): the bar tooltip renders in <body> and flips below when there is no room above (#3264)

[dolho]

Summary

• On the Workspace agent band the activity chart is 23px tall, sits just under the page header, and is inside ScanlineReveal's clip-path. The bar tooltip was an absolute bottom-full child of the bar, so it always opened upward and its top rows (the date, the top buckets) were covered.
• StackedBarChart now renders one tooltip through <Teleport to="body"> with position: fixed and z-50, placed from the hovered bar's rect. No parent can clip or cover it.
• Placement: above when it fits; otherwise below when that fits; otherwise whichever side has more room. Horizontally it is centred on the bar and clamped 8px inside the viewport.
• It stays hidden until measured, so it never flashes at 0,0. It follows the bar through scroll (capture) and resize, and the listeners are removed on leave and on unmount.
• Markup and classes are unchanged (the same dark chip, gray only). All callers get the fix: the Workspace band, the agent Overview and canvas charts.

Changes

• src/frontend/src/components/StackedBarChart.vue
• src/frontend/tests/unit/stackedBarChartTooltip.mount.spec.js (new mount spec, jsdom)

Test Plan

• npx vitest run tests/unit/stackedBarChartTooltip.mount.spec.js: 6 passed, and all 6 fail against the old component. The cases:
  • the tooltip renders in <body>, outside the chart;
  • it opens above when there is room;
  • it flips below near the top;
  • it is clamped at the left edge;
  • it is clamped at the right edge;
  • an empty day shows nothing, and the tooltip goes away on leave.
• Full frontend unit suite: 263 files, 4,649 tests passed, including the raw-colour, source-text and loading-gate ratchets. npm run check:tokens OK.
• Playwright on a local instance, Workspace → sales-companion, light and dark: hovering a bar shows the full tooltip (date, buckets, total) drawn over the header, not under it.
• Human visual check (reviewer, on a local instance): Workspace band and agent Overview, light and dark, plus the edge bars.

Fixes #3264

🤖 Generated with Claude Code

[vybe]

merge-train: batch validated on train/20261006-1038 (#3270)