fix(dashboards): unify timeseries loading and empty state

[nikkikapadia]

aligned the heatmap loading state with the timeseries loading state (aka just the spinner and no loading placeholder shadow) and the empty state with the state of all the other widgets.

ALSO good to note that all non-timeseries widgets have that loading screen; wondering if we want to change them all just to unify the UI.
but anyways this is what it looks like:

Before	After

[linear-code]

DAIN-1775

[sentry]

Bug: The WidgetLoadingPanel for heatmaps lacks a positioned parent (position: relative) during reloads, causing the loading indicator to be displayed incorrectly outside the chart area.
_{Severity: MEDIUM}

Suggested Fix

Add position: relative; to the ChartWrapper styled-component. This will establish a positioning context for the absolutely positioned WidgetLoadingPanel, ensuring it is correctly constrained within the chart's boundaries during loading states.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: static/app/views/dashboards/widgetCard/chart.tsx#L467-L468

Potential issue: When a heatmap widget reloads data, the `HeatmapSeriesComponent`
renders a `WidgetLoadingPanel`. This panel uses `position: absolute` styling for its
layout. However, it is rendered inside a `ChartWrapper` component that lacks a
`position: relative` (or similar) style property. Because an absolutely positioned
element is positioned relative to its nearest positioned ancestor, the loading panel
will not be constrained to the chart's area. This results in the loading indicator being
displayed incorrectly, potentially overlaying other UI elements outside of the widget's
boundaries.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[nikkikapadia]

hm im not seeing this issue visually so i think it's fine? let me know if yall are seeing this

[gggritso]

Beaut