feat(events): route all backend events through a single event bus and store

Adds src/services/bus (the only listen/emit site: one handler per contract
event plus the frontend-internal test-all event) and the React
context/reducer app store. Migrates the five scattered subscriptions
(layout events, pending failures, proxy refresh, profile update/timer,
test-all), deletes the old glue (events.ts, use-listen, use-layout-events,
LoadingCacheProvider), moves run state into the store, and adds an
architecture gate script wired into CI. Behavior is unchanged; the
run-state query's poll/focus safety net is preserved by the store
provider.

package.json

@@ -31,6 +31,7 @@
     "i18n:format": "node scripts/cleanup-unused-i18n.mjs --align --apply",
     "i18n:types": "node scripts/generate-i18n-keys.mjs",
     "gen:contract": "node scripts/gen-contract.mjs",
+    "arch:check": "node scripts/check-architecture.mjs listen emit invoke",
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",