feat(brainstorm-companion): resilient reconnect, live status, paused overlay

The injected client reconnected on a fixed 1s timer with no feedback: if the
laptop slept or the server restarted, the page showed 'Connected' over a dead
socket and silently queued events. And when the server stopped, the user got a
bare connection-refused with no explanation.

helper.js now:
- reconnects with exponential backoff (500ms, doubling, capped at 30s; reset on
  open), with an onerror->close handler, nulls the socket on close, and clears a
  pending timer before scheduling another;
- drives the frame status pill Connected/Reconnecting/Disconnected via a
  --status-color custom property (frame-template.html);
- after ~15s disconnected, shows a self-styled 'Companion paused' overlay
  (tombstone) explaining the companion stopped and will reconnect automatically;
- on recovery from a tombstoned outage (e.g. server restarted on the same port)
  reloads to pick up the restarted server's current screen.

The reconnect-backoff is an exported pure function; helper.test.js unit-tests it
(doubling + cap progression) and asserts the status/tombstone/reconnect wiring.
DOM behaviour is verified live.

Refs #856, #1237

tests/brainstorm-server/package.json

@@ -2,7 +2,7 @@
   "name": "brainstorm-server-tests",
   "version": "1.0.0",
   "scripts": {
-    "test": "node ws-protocol.test.js && node server.test.js && node lifecycle.test.js && bash stop-server.test.sh"
+    "test": "node ws-protocol.test.js && node helper.test.js && node server.test.js && node lifecycle.test.js && bash stop-server.test.sh"
   },
   "dependencies": {
     "ws": "^8.19.0"