Fix line chart tooltip stacking above the header and navigation on scroll

[sileshidev-lab]

Summary

Fixes #13235 — clicking a point on the line chart in "Find out how your audience is growing" pins its tooltip open; scrolling then moves the tooltip up over the sticky header and navigation chips instead of behind them.

#4360 and #4496 fixed this same class of bug for the other dashboard charts (PieChart tooltip), via PR #4549, which added z-index: 8; to the PieChart tooltip selector in three SCSS files. The .googlesitekit-chart--LineChart div.google-visualization-tooltip selector — sitting right next to the PieChart one in the same file — never got that treatment, so the line chart's tooltip (used by the "audience growing" widget's UserCountGraph) was left with no z-index at all.

Fix: add the same z-index: 8; to the LineChart tooltip selector in _googlesitekit-chart.scss, matching the existing PieChart rule immediately below it. Confirmed the header's z-index (11, in _googlesitekit-header.scss) is higher, so this correctly puts the tooltip behind it.

Verification

• Confirmed via the Storybook Loaded story for DashboardAllTrafficWidgetGA4 (which renders the UserCountGraph line chart with real data) that pinning a tooltip and inspecting the live rendered element shows z-index: 8 actually applied — not just present in source, but compiled and loaded in the browser
• npx stylelint on the changed file — clean
• No new Storybook stories/VRT needed — this fixes stacking behavior on an already-covered chart selector, same as the precedent PR (Fix graph widget tooltip overlap with header #4549), which also didn't add new stories

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[sileshidev-lab]

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