fix(ui): stop the game-screen freeze when an opponent joins (reactive self-loop)

The in-game live-event $effect read `view`/`moves`/`placement` (via cacheSnapshot
and direct reads) AND wrote `view` in its branches, so those reads became the
effect's own dependencies: writing `view = …` re-ran the effect, and with
app.lastEvent unchanged it re-entered the same branch and wrote `view` again — a
tight self-invalidating loop that pinned the main thread, freezing the board and
rack. opponent_moved escaped it only because applyMoveDelta is idempotent on the
move count (no cache → no write on the second pass); opponent_joined and game_over
have no such guard, so an opponent joining hung the whole screen. Tracking
`placement` similarly re-fired the handler on every tile the player placed after
an opponent's move (a spurious reload).

Fix: the effect must depend only on app.lastEvent and process each event once.
Wrap the branch body in `untrack`, scoping its view/moves/placement reads out of
the effect's dependency set; the writes inside no longer re-trigger it.

e2e: after the opponent joins, placing a rack tile must render a pending tile —
verified RED (frozen, 0 pending tiles, both engines) before the fix, GREEN after.

ui/e2e/quickmatch.spec.ts

@@ -76,3 +76,17 @@ test('quick game: a foreground resync recovers a join shed while the stream stay
   await expect(page.getByText('Robo')).toBeVisible();
   await expect(page.getByText(/Searching for opponent/)).toHaveCount(0);
 });
+
+// Regression: an opponent joining must not freeze the screen. The in-game live-event effect tracked
+// the `view` it also writes, so opponent_joined re-invalidated itself in a tight loop (opponent_moved
+// was spared only by its delta's move-count idempotency). The board must still respond afterwards.
+test('quick game: the board stays interactive after the opponent joins', async ({ page }) => {
+  await enterOpenGame(page);
+  await page.evaluate(() => (window as unknown as { __mock: { joinOpponent(): void } }).__mock.joinOpponent());
+  await expect(page.getByText('Robo')).toBeVisible();
+
+  // Place a rack tile on the centre star: a frozen screen renders no pending tile.
+  await page.locator('.rack .tile').first().click();
+  await page.locator('[data-cell]:not(.filled)').nth(112).click(); // row 7, col 7
+  await expect(page.locator('[data-cell].pending')).toHaveCount(1);
+});