#630 - prune lingering y-axis when deselecting topic in multi-axis mode

[bracyw]

Changes

With Multiple Y-Axes enabled, the graph page's y-axis configs are now rebuilt in updateChart from the current data map, so the axis set always matches the selected topics. Previously the multi-y-axis effect only re-ran when the showMultipleYAxes toggle flipped — deselecting a topic dropped its series but left its y-axis behind, and axes accumulated as topics were toggled. The axis-building logic is factored into a shared applyMultiYAxisConfigs helper used by both the toggle effect and updateChart; the read in updateChart is untracked so it doesn't add showMultipleYAxes as a dependency of the subscription effect.

Notes

Single-axis behavior is untouched — its min/max is still owned by the graphConfig effect, so that path is left alone.

Local live verification with verify-graph could not be run in this environment (no Docker daemon → no backend/telemetry data). Verified via build, lint/prettier, and a unit spec driving the axis seam. verify-graph should be run on a Docker-capable machine before merge.

Test Cases

• Multi-axis on: one y-axis per topic in the data map.
• Deselect a topic: its y-axis is pruned, remaining axes match remaining series.
• Axis sides alternate left/right as topics are added.

Checklist

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Closes #630

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