[feature]: Show estimate point totals in list view group headers

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Summary

Display the sum of estimate points next to each group's work item count in the list view header. This is opt-in via a new "Show estimate totals" toggle in Display Filters → Extra Options (same pattern as "Show sub-work items").

Why should this be worked on?

When using point or time estimates with the list layout grouped by any property (cycle, status, priority, assignee, etc.), there's no way to see the total estimated effort per group at a glance. You have to mentally sum the individual work item estimates, which breaks down quickly with larger groups.

Presenting estimate totals to the user is a standard capability in tools like Jira (shows estimate sums in sprint headers and swimlane headers) and Linear (shows point totals on project views). It's essential for sprint planning, because the team needs to quickly assess whether a sprint/group is over-allocated or under-allocated without opening analytics. If there is a "plane-native" way to do this in a different way, we have not found it yet.

A related feature request #8652 (estimate visibility on epics) was shipped in v2.4.0, demonstrating demand for better estimate visibility. This extends that pattern to a common daily workflow: the list view.