Handle matchmaking socket close codes per API contract (#4595)

## What

Implements the close-code contract from the matchmaking API handoff (API
PR #419) in the matchmaking modal, plus two runnable integration
harnesses. Previously `onclose` only logged, so an API deploy while
queued left the player on the "searching" spinner forever (the queue is
in-memory on the API worker).

| Close | Behavior |
| --- | --- |
| 1008 `Invalid session` | Reconnect and re-send `join` —
`getPlayToken()` refreshes an expired token internally, so the rejoin
carries a fresh JWT |
| 1000 `Replaced by newer connection` | Another tab/window took the
queue slot: show a message (new `matchmaking_modal.replaced` string) and
stop. No retry |
| Any other close before assignment | Server restart/deploy: reconnect
and re-send `join`, with exponential backoff (1s doubling to a 15s cap)
|

Intentional closes (user backs out of the modal, assignment received)
don't reconnect. A pending 2s join timer from a previous socket is
cleared before each reconnect so it can't fire on the new socket.

## Test harnesses (`tests/matchmaking/`)

- **`npm run test:matchmaking`** (contained): drives the real modal in a
headless browser against a fake in-process matchmaking server speaking
the documented protocol; covers the whole close-code table over real
WebSockets. Needs only `npm run dev`.
- **`npm run test:matchmaking:e2e`**: real integration against the API
worker on `localhost:8787` — two browser players join the real queue,
the dev game server receives the checkin assignment, and both clients
end up in the same created game.

## Why now

This is required by the matchmaking API's client contract independent of
the upcoming 2v2 work ("clients must already handle unexpected close →
reconnect and rejoin"). The rest of the 2v2 integration is planned for
the next version.

## Verification

- Contained harness: 8/8 checks pass (join, 1006 reconnect+rejoin, 1008
rejoin with fresh token, assignment, replaced → message + no retry,
intentional close → no retry/no message).
- E2E harness against a local worker: 3/3 — both players matched into
the same game, game created via checkin and joinable by both.
- `npm test` (all 2,046 pass), `npx tsc --noEmit`, ESLint clean.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

resources/lang/en.json

@@ -1074,6 +1074,7 @@
     "connecting": "Connecting to matchmaking server...",
     "elo": "Your ELO: {elo}",
     "no_elo": "No ELO yet",
+    "replaced": "You joined matchmaking from another tab or window.",
     "searching": "Searching for game...",
     "title": "1v1 Ranked Matchmaking (ALPHA)",
     "waiting_for_game": "Waiting for game to start..."