feat(system): native Overview with health report and statistics

System > Overview renders the concept's metrics and its Library health and
Workspace preferences surfaces from GET /system/overview: real counts,
"Not checked" until a health check ran, no invented review count, backup
or thumbnail status (D93). Explore opens "Usage statistics" with the real
runs and ratings per category and the Library totals (D96); the
statistics route now also returns rated_images.

Maintenance shows the concept's notice with Run health checks above its
hand-off: the read-only check runs as a registry read job and opens the
"Health report" with a chip per real result, missing files per source
(D95). A load failure shows Retry and no zeros; late answers never
repaint a newer section.

Adds the System concept comparison (shell, Overview, both dialogs, the
notice, mutation checks), the System workflow spec and its fixture
hooks, and the TESTING.md "Native System" section.

web/js/workspace/workspace.css

@@ -89,6 +89,8 @@ main:focus-visible { outline: 2px solid var(--ca-focus); outline-offset: 4px; }
    not save is highlighted, as its "Review the highlighted settings" status says. */
 .save-row button[aria-disabled="true"] { opacity: .55; cursor: not-allowed; /* D92 */ }
 .setting-row [aria-invalid="true"] { outline: 2px solid var(--red); outline-offset: 2px; /* D92 */ }
+/* D93: an Overview row whose action is not available yet (the review queue, Q6) keeps its button, disabled. */
+.operation .row-actions button[aria-disabled="true"] { opacity: .55; cursor: not-allowed; /* D93 */ }
 @media (max-width: 700px) {
     .stat-grid { gap: 8px; }
     .stat { padding: 16px; }