feat(dashboard): [#305] added axis labels, hover tooltips, and load-in animation to metric graphs

[AaronProbha18]

Fixes: #305

Changes

• Extended renderLineChart() in fenn/dashboard/static/app.js to draw min/max axis tick labels (step on the x-axis, value on the y-axis) as SVG <text> elements, and added a left (value) axis line alongside the existing baseline (step) axis. Runs for every .chart-card uniformly, so both inline session-page cards and the new metric detail page get labeled axes with no per-caller changes.
• Added attachChartHover() to app.js, binding pointermove/pointerleave once per <svg> (guarded by a dataset.hoverBound flag so repeated re-renders from auto-refresh or tab switching don't stack duplicate listeners). On move, it finds the nearest point by step and shows a tooltip with the exact step/value at the pointer.
• Added an options.animate parameter to renderLineChart(): when set, the polyline draws in via a stroke-dasharray/stroke-dashoffset CSS transition instead of appearing immediately. renderSessionGraphs() now reads card.dataset.animate and passes it through, so only cards explicitly opted in animate.
• Set data-animate="true" on the chart card in session_metric_detail.html only — inline session-page cards are left unset, so they render immediately and aren't replayed every 5s by the running-session auto-refresh poll.
• Added .chart-axis-label and .chart-tooltip styles, and a .chart-card-detail .chart-svg height override for the larger detail-page chart, to fenn/dashboard/static/style.css.