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/dashboard.json

@@ -18,5 +18,94 @@
   "failedConnect": "Failed to connect to server",
   "failedLoad": "Failed to load data",
   "activeTrend": " active",
-  "totalTrend": " in active sessions"
+  "totalTrend": " in active sessions",
+  "tabs": {
+    "label": "Dashboard views",
+    "analytics": "Analytics",
+    "monitor": "Monitor",
+    "health": "Health"
+  },
+  "collapseSubagents": "Collapse subagents",
+  "expandSubagents": "Expand subagents",
+  "needsYou": {
+    "title": "Needs you",
+    "since": "waiting {{ago}}"
+  },
+  "health": {
+    "runtime": "Runtime",
+    "cores_one": "{{count}} core",
+    "cores_other": "{{count}} cores",
+    "uptime": "Uptime",
+    "cpu": "CPU (1/5/15m)",
+    "nodeRss": "Node RSS",
+    "hostMemory": "Host Memory",
+    "hostMemoryTip": "Host Memory: {{pct}}% used\n{{used}} / {{total}}",
+    "heap": "V8 Heap",
+    "heapTip": "V8 Heap: {{pct}}% used\n{{used}} / {{limit}} limit\nAllocated: {{allocated}}\nExternal: {{external}}",
+    "storage": "Storage",
+    "writeVelocityTip": "Write velocity (events):\n5 min: {{m5}}\n15 min: {{m15}}\n1 hr: {{h1}}",
+    "database": "Database",
+    "recordsTip": "Record distribution\n\nSessions: {{sessions}} ({{sessPct}}%)\nAgents: {{agents}} ({{agentPct}}%)\nEvents: {{events}} ({{eventPct}}%)\n\nTotal: {{total}} records",
+    "total": "total",
+    "sessions": "Sessions",
+    "agents": "Agents",
+    "events": "Events",
+    "score": "Health Score",
+    "formula": "Formula",
+    "formulaTip": "Composite index formula:\n0.4 × Success rate ({{success}}%)\n+ 0.25 × Cache hit ({{cache}}%)\n+ 0.25 × (100 − Error rate) ({{noError}}%)\n+ 0.10 × (100 − Heap%) ({{noHeap}}%)\n= {{score}}",
+    "scoreTip": "Score: {{score}} / 100\n\n• Success rate (40%): {{success}}%\n• Cache hit (25%): {{cache}}%\n• Error avoidance (25%): {{noError}}%\n• Memory health (10%): {{noHeap}}%",
+    "cache": "Cache",
+    "cacheTip": "Cache hit rate: {{pct}}%\nHits: {{hits}}\nMisses: {{misses}}",
+    "errors": "Errors",
+    "errorsTip": "Error rate: {{pct}}%\n<5% = healthy, 5–15% = warning, >15% = critical",
+    "compact": "Compact",
+    "compactTip": "Transcript compactions: {{count}}\nReduces the context window by summarizing turns.",
+    "saved": "Saved",
+    "savedTip": "Tokens recovered: {{tokens}}\nFreed by compaction.",
+    "tokenUsage": "Token Usage",
+    "totalSuffix": "{{value}} total",
+    "modelTip": "{{model}}\nInput: {{input}}\nOutput: {{output}}\nCache read: {{cacheRead}}\nShare: {{pct}}%",
+    "noModelData": "No model data",
+    "concurrency": "Concurrency",
+    "intervals_one": "{{count}} interval",
+    "intervals_other": "{{count}} intervals",
+    "concurrencyTip": "Peak: {{peak}} parallel sessions\nActive intervals: {{active}}\nAvg: {{avg}}",
+    "noConcurrencyData": "No concurrency data",
+    "peak": "Peak",
+    "peakTip": "Peak: {{count}} sessions running simultaneously.",
+    "active": "Active",
+    "activeTip": "{{active}} of {{total}} intervals have active sessions.",
+    "avg": "Avg",
+    "avgTip": "Average concurrency across all intervals.",
+    "toolUsage": "Tool Usage",
+    "top": "top {{count}}",
+    "invocationsTip_one": "{{tool}}: {{formatted}} invocation",
+    "invocationsTip_other": "{{tool}}: {{formatted}} invocations",
+    "noToolData": "No tool data yet",
+    "subagentEffectiveness": "Subagent Effectiveness",
+    "defaultType": "default",
+    "effectivenessTip": "{{type}}\nSuccess: {{pct}}%\nTotal: {{total}} · OK: {{ok}} · Errors: {{errors}}",
+    "noSubagentData": "No subagent data yet",
+    "integration": "Integration",
+    "hooksActive": "Active",
+    "hookTip": "Path: {{path}}\nStatus: {{status}}",
+    "connected": "Connected",
+    "disconnected": "Disconnected",
+    "noHooks": "No project hooks registered",
+    "wsTip": "WebSocket connections: {{count}}\nProtocol: RFC 6455",
+    "wsActive": "WebSocket active",
+    "connections_one": "{{count}} connection",
+    "connections_other": "{{count}} connections",
+    "platform": "Platform",
+    "journalMode": "Journal mode",
+    "synchronous": "Synchronous",
+    "autoVacuum": "Auto-vacuum",
+    "foreignKeys": "Foreign keys",
+    "busyTimeout": "Busy timeout"
+  },
+  "errorBoundary": {
+    "title": "Something went wrong",
+    "body": "This page failed to load. Reloading usually fixes it — for example after the dashboard was updated.",
+    "reload": "Reload"
+  }
 }