Add purpose-built empty and error states to DAP Impact panels

What does this MR do and why?

DAP Impact dashboard panels get purpose-built states in place of the bare "Something went wrong" span and hover-only popover: no data, no access, ClickHouse configuration required, Siphon data not available yet, a retryable error with a working inline Try again, and a non-retryable error. A new shared panel_state.vue renders these in full and compact layouts, a new GLQL error classifier maps error extensions codes and /api/glql HTTP statuses to states, and retry now genuinely re-runs the query; the kebab Reload had the same no-op bug and is fixed the same way. The GLQL resolver keeps its parsed config through a failed execution so error states can size themselves by display type, and keeps already-loaded rows visible when a continuation page fails.

Every GLQL panel rendered through analytics_dashboard_panel.vue inherits these states, including user-defined custom dashboards. The dashboard YAML also re-enables the default panel actions menu on all DAP panels, as agreed with design in Slack, and both visualization schemas gain the emptyState key for per-panel empty-state copy. analytics_dashboard_panel_spec.js is unchanged because GLQL panels no longer route errors through the panel's alert mechanism; the new coverage lives in glql_spec.js and panel_state_spec.js.

Stacked on !256769 (merged).

References

• Resolves https://gitlab.com/gitlab-org/gitlab/-/work_items/629384
• Covers the inline-error intent of #604173 for GLQL panels.

How to set up and validate locally

1. Sign in as a user with the Reporter role on a group with DAP data and open /explore/analytics_dashboards/dap_impact; the credits panels show the access state, the rest load.
2. Pick a group without DAP activity for the no-data state.
3. Stop Siphon or ClickHouse locally for the availability states.

Screenshots or screen recordings

State	Before	After
No access (Reporter)
No data
Siphon unavailable
Generic error
After Try again