COD-131 sync tab order across devices via server state

Tab reordering (drag-and-drop + Ctrl+Shift+{/}) persisted only to
localStorage (codeman-session-order), so each device kept its own private
order. Add server-side persistence so the order follows the user across
devices, live. Takes the issue's recommended default (a): one global order,
server authoritative, localStorage as offline fallback.

- session-order.ts (new, pure + unit-tested): normalizeSessionOrder (coerce
  to string[], drop empty/non-string, dedup) and mergeSessionOrder (the
  pushing device's order wins; ids the device hadn't loaded fall to the end
  in their existing relative order, never dropped — graceful for
  closed/remote/parked sessions absent on that device).
- AppState.sessionOrder?: string[]; StateStore get/setSessionOrder + the field
  added to buildPartialJson() (the incremental serializer whitelists fields,
  so without this the value never reached disk / survived a restart).
- PUT /api/session-order (session-routes): parse -> merge -> persist ->
  broadcast session:orderChanged; getLightState() init snapshot now carries
  sessionOrder so a fresh load/reconnect restores it.
- SSE event session:orderChanged registered in sse-events.ts + constants.js.
- app.js: handleInit seeds localStorage from the server snapshot before
  syncSessionOrder(); saveSessionOrder() also PUTs to the server (debounced
  400ms, covers drag + both keyboard moves); _onSessionOrderChanged adopts a
  remote order and re-renders (no-op-guarded to avoid echo flicker).

Verified (orchestrator re-ran all gates): tsc 0, lint 0, frontend-syntax +
prettier clean, build ok; session-order + session-order-routes + state-store
56/56. Functional round-trip on an isolated beta: PUT {a,b,c} -> status
snapshot reflects it; merge PUT {c,a} vs {a,b,c} -> {c,a,b} (b preserved at
end); malformed payload rejected with a clean 400; sessionOrder persisted to
state.json and survived a restart.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

(cherry picked from commit 79415f2fdfbdf3fbe362a063534e7f84c553eefb)

src/web/routes/session-routes.ts

@@ -35,7 +35,9 @@ import {
   QuickRunSchema,
   QuickStartSchema,
   InteractiveStartSchema,
+  SessionOrderUpdateSchema,
 } from '../schemas.js';
+import { mergeSessionOrder } from '../../session-order.js';
 import {
   autoConfigureRalph,
   CASES_DIR,
@@ -280,6 +282,18 @@ export function registerSessionRoutes(
     return ctx.getLightSessionsState();
   });
 
+  // ========== Session Tab Order (global sync, COD-131) ==========
+
+  app.put('/api/session-order', async (req): Promise<ApiResponse<{ order: string[] }>> => {
+    const { order } = parseBody(SessionOrderUpdateSchema, req.body, 'Invalid session order');
+    // Server is authoritative but never drops ids it knows about that the
+    // pushing device hadn't loaded yet — those fall to the end (mergeSessionOrder).
+    const merged = mergeSessionOrder(order, ctx.store.getSessionOrder());
+    ctx.store.setSessionOrder(merged);
+    ctx.broadcast(SseEvent.SessionOrderChanged, { order: merged });
+    return { success: true, data: { order: merged } };
+  });
+
   // ========== Session Creation ==========
 
   app.post('/api/sessions', async (req) => {