feat(dashboard): lazy routes, Needs-you strip, URL state, exact numbers

- Workspace, SettingsHub, SessionDetail and the Workflow sub-tab are lazy
  (main bundle 2,166 kB -> ~920 kB), wrapped in RouteErrorBoundary: a stale
  chunk after a self-update reloads once (vite:preloadError too), any other
  error shows a Reload panel instead of a blank page.
- Live refresh: lib/throttle.ts + hooks/useLiveRefresh.ts replace trailing
  debounces that froze under a steady event stream; refresh skips hidden
  tabs and refetches after a WebSocket reconnect. Monitor loads only while
  shown, with one batched GET /api/agents?session_ids= instead of N+1; a
  failing endpoint is an inline error, not a blank Dashboard.
- NeedsYouStrip on Monitor and above Analytics. Tabs, filters, page, sort
  and search live in the URL (Dashboard, Sessions, Activity, SessionDetail).
- Sessions: tool / workspace / date-range filters, CSV/JSON export
  (formula-neutralised), focusable row links, Open in workspace (the dead
  /run links are gone), stale responses dropped, error + Retry.
- Fixes: "NaNs" durations, 30/7-day charts skipping idle days, weekday
  labels west of UTC (now averages), cache hit rate denominator, billable
  tokens vs cache reads, exact tool-call counts, "To" date = end of day,
  invalid ?from= no longer throws, recent list no longer shrinks to 15,
  stable message keys, transcript switch races.
- Light-theme-unreadable bg-black code panels and dark-only DAG fills use
  tokens; status configs and chart colours use theme tokens; Health tab
  and other literals translated.
- Server (additive): GET /api/agents?session_ids=, GET /api/sessions
  awaiting=1&from=&to=, stats tool_call_count/distinct_tool_count, event
  rows carry session/agent labels.

client/src/i18n/locales/en/workflows.json

@@ -204,6 +204,15 @@
         "outcomeAbandoned": "Sessions that stopped without a clean Stop event and without explicit errors - usually closed mid-flight.",
         "outcomeFallback": "The terminal status of sessions in scope, computed from the last hook event observed for each session."
       }
+    },
+    "legendItems": {
+      "sessions": "Sessions",
+      "mainAgent": "Main agent",
+      "subagentTypes": "Subagent types",
+      "compactions": "Compactions",
+      "completed": "Completed",
+      "error": "Error",
+      "abandoned": "Abandoned"
     }
   },
   "drillIn": {
@@ -230,7 +239,9 @@
     "noEvents": "No events recorded for this session.",
     "showingOf": "Showing 100 of {{total}} events",
     "sessionInfo": " session(s) - active {{from}}%-{{to}}% of session",
-    "sortedByCompaction": " session(s) (sorted by compaction count)"
+    "sortedByCompaction": " session(s) (sorted by compaction count)",
+    "openSession": "Open session",
+    "openInWorkspace": "Open in workspace"
   },
   "lanes": {
     "title": "Workspace Pipeline Pass Rates",
@@ -316,5 +327,7 @@
     "sessions_one": "{{count}} session",
     "sessions_other": "{{count}} sessions",
     "pluginBreakdown": "{{skills}} skill(s) used · {{skillCalls}} skill call(s) · {{mcpCalls}} MCP tool call(s)"
-  }
+  },
+  "selectedNode": "Selected:",
+  "clearSelection": "Clear selection"
 }