fix(lobby): clear stale MP creds on Play Solo to avoid blank-screen trap

If the user played multiplayer earlier in the tab session, room-code,
room-token, and player-color persisted in sessionStorage. Clicking Play
Solo then:

  1. navigate('/game')  — no code param
  2. GameRoute reads sessionStorage, finds stale creds → Case 1
     canonicalises the URL to /game/<stale-code>
  3. MultiplayerGameView mounts, opens a WS to a dead room, handshake
     fails silently → blank white screen with a live URL like
     /game/OSJBJY in the address bar.

Fix: handlePlaySolo explicitly wipes room-code, room-token, player-color,
layout-name, and modifier-profile-name before navigating. The solo path
then goes through GameRoute's Case 2 (no code, no creds) and mounts
GameView cleanly.

Regression test in solo-smoke.spec.ts seeds sessionStorage with stale
MP creds, clicks Play Solo, and asserts:
  - URL settles on /game (not /game/<stale>)
  - No 'mp-joining' placeholder
  - Board renders (e2 pawn visible)
  - All stale keys are wiped from sessionStorage
  - No console errors

Verified the test fails without the fix (Playwright hits the blank
screen / Joining placeholder) and passes with it.

packages/chess/e2e/solo-smoke.spec.ts

@@ -209,4 +209,70 @@ test.describe('Solo-play smoke (T2 preview tests)', () => {
       timeout: 3000,
     });
   });
+
+  // Regression guard for the "play-solo lands on blank screen" bug.
+  //
+  // Before the fix, a stale multiplayer session (room-code/room-token in
+  // sessionStorage from a previous room.create) would make Play Solo
+  // navigate to /game → GameRoute's Case 1 canonicalises to
+  // /game/<stale-code> → MultiplayerGameView mounts → WS handshake to a
+  // long-dead room silently fails → blank screen.
+  //
+  // The fix: handlePlaySolo() explicitly clears room-code, room-token,
+  // player-color, layout-name, and modifier-profile-name from
+  // sessionStorage before navigating, guaranteeing a fresh solo mount.
+  test('play-solo with stale MP creds in sessionStorage lands on /game, not /game/<stale>', async ({
+    page,
+  }) => {
+    // Seed sessionStorage to simulate a previous room.create or room.join
+    // whose creds were never cleared (e.g. user closed the tab after
+    // playing multiplayer, then reopened the lobby later).
+    await page.evaluate(() => {
+      sessionStorage.setItem('room-code', 'STALE1');
+      sessionStorage.setItem('room-token', 'stale-token-deadbeef');
+      sessionStorage.setItem('player-color', 'white');
+      sessionStorage.setItem('layout-name', 'Some Previous Layout');
+      sessionStorage.setItem('modifier-profile-name', 'Old Profile');
+    });
+
+    const errors: string[] = [];
+    page.on('pageerror', (e) => errors.push(`PAGE: ${e.message}`));
+    page.on('console', (msg) => {
+      if (msg.type() === 'error') errors.push(`CONSOLE: ${msg.text()}`);
+    });
+
+    await page.locator('[data-action="play-solo"]').click();
+
+    // URL must settle on /game WITHOUT a code suffix. Using a short
+    // timeout so a regression surfaces as a test failure rather than
+    // the suite hanging on the blank-screen state.
+    await page.waitForURL('**/game', { timeout: 3000 });
+    await expect(page).toHaveURL(/\/game$/);
+
+    // Board must render a real solo game (e2 pawn present, not the
+    // "Joining room …" placeholder).
+    await expect(page.locator('[data-testid="mp-joining"]')).toHaveCount(0);
+    await expect(page.locator('[data-square="e2"] [data-piece]')).toBeVisible({
+      timeout: 3000,
+    });
+
+    // All stale sessionStorage entries must be wiped so a page reload
+    // doesn't re-trigger the bug on the refreshed /game route.
+    const staleCreds = await page.evaluate(() => ({
+      code: sessionStorage.getItem('room-code'),
+      token: sessionStorage.getItem('room-token'),
+      color: sessionStorage.getItem('player-color'),
+      layout: sessionStorage.getItem('layout-name'),
+      profile: sessionStorage.getItem('modifier-profile-name'),
+    }));
+    expect(staleCreds).toEqual({
+      code: null,
+      token: null,
+      color: null,
+      layout: null,
+      profile: null,
+    });
+
+    if (errors.length > 0) throw new Error(errors.join('; '));
+  });
 });

packages/chess/src/ui/Lobby.tsx

@@ -324,6 +324,17 @@ export function Lobby({ chessState }: LobbyProps = {}) {
   };
 
   const handlePlaySolo = () => {
+    // Clear any stale multiplayer creds from a previous session. Without
+    // this, a user who played multiplayer earlier (creds still in
+    // sessionStorage) would hit GameRoute's Case 1 — /game with creds
+    // present gets canonicalised to /game/<stale-code>, which then
+    // tries to reconnect to a long-dead room and renders a blank
+    // screen when the WS handshake silently fails.
+    sessionStorage.removeItem('room-code');
+    sessionStorage.removeItem('room-token');
+    sessionStorage.removeItem('player-color');
+    sessionStorage.removeItem('layout-name');
+    sessionStorage.removeItem('modifier-profile-name');
     resetToFreshGame();
     navigate('/game');
   };