feat(ui): maintainer dashboard panel for per-tool MCP usage counts

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Context

Part of #6228. Once telemetry data exists (companion issues), this issue builds the consumption side: a dashboard view so a maintainer can actually see which MCP tools get used, by whom (local vs. remote), and how often.

Requirements

• Add a new panel to apps/loopover-ui (following the existing app-panels/ convention — maintainer-panel.tsx or a new sibling panel) showing per-tool call counts, success/failure rates, and a local-vs-remote split, over a selectable time window.
• The panel should be buildable and testable against a defined mock data shape even before real production data volume exists — do not block this issue on the telemetry-writing issues actually shipping to production first, only on their data shape being defined (see Spec: usage-telemetry instrumentation strategy for MCP (PostHog) #6228 and the wrapper-module issues for the exact event schema).
• Follow this repo's existing dashboard conventions found by this session's UI audit — AnalyticsCardShell for card chrome, RefreshMeta for staleness, errorKind-aware StateBoundary for error states (see fix(ui): adopt AnalyticsCardShell in 5 analytics cards that hand-roll their own chrome (fixes CycleTimeCard's broken empty state) #6175/fix(ui): content-shaped loadingSkeleton (#793) adopted in only 1 of several list/table StateBoundary sites, causing layout jump #6178/fix(ui): config generator never mounts ConfigGeneratorYamlPreview, so it never shows the user the YAML it generates #6183 for the exact patterns now that they're fixed).
• This is a UI-only change under apps/**.

Test Coverage Requirements

Standard UI test conventions for this codebase; mock the usage-data API response shape.

Deliverables

• A new panel showing per-tool MCP usage, reachable from the maintainer dashboard.

Expected Outcome

A maintainer can see real MCP usage data instead of it only existing in PostHog's own external dashboard.

Links & Resources

• Spec: usage-telemetry instrumentation strategy for MCP (PostHog) #6228 (parent spec, defines the event schema this panel consumes)
• apps/loopover-ui/src/components/site/app-panels/maintainer-panel.tsx (dashboard convention to follow)

[linear-code]

JSO-4329