fix(frontend): scope player identity to a tab, not the browser

Two tabs of the same browser share localStorage, so both got the same
persisted playerId and stepped on each other: matchmaking's waiting
queue is keyed by playerId and dedupes a second queue:join as a no-op,
so two tabs searching never both count as waiting; and re-identifying
with the same playerId hijacks the ws registry's socket binding, tearing
down the first tab's connection (surfacing as room:join replying 'room
already has two players' once its creator's socket is dead).

Move playerId/name from localStorage to sessionStorage: still stable
across reloads within a tab (reconnect-by-refresh keeps the same
identity), but each new tab gets a fresh UUID, so two tabs of one
browser behave like two separate players. Not IP-related; no such
restriction existed.

backend/src/PROTOCOL.md

@@ -25,8 +25,10 @@ message-specific fields — no envelope wrapper, no request/response ids.
 
 There is no account system and none is planned — adding auth is out of
 scope for multiplayer entirely. Instead, the client generates a
-`playerId` (a `crypto.randomUUID()` v4 string) the first time it loads,
-persists it in `localStorage`, and reuses it on every future visit. The
+`playerId` (a `crypto.randomUUID()` v4 string) the first time a tab
+loads, persists it in `sessionStorage`, and reuses it for the lifetime
+of that tab (a reload keeps the same identity; a new tab gets a new
+one, so two tabs of the same browser act as two distinct players). The
 player also picks/keeps a display `name` (also persisted client-side, no
 server-side uniqueness check). The very first message a client sends
 after the socket opens registers both with the server:

frontend/src/net/identity.ts

@@ -1,18 +1,21 @@
-// Per-browser player identity. There is no account system (see
+// Per-tab player identity. There is no account system (see
 // backend/src/PROTOCOL.md's "Identity" section) — just a stable UUID
 // generated once and reused, plus a freely editable display name. Both
-// are persisted in localStorage so they survive reloads and reconnects.
+// are persisted in sessionStorage so they survive reloads/reconnects
+// within a tab but don't leak across tabs — two tabs of the same
+// browser (e.g. testing matchmaking/private rooms locally) are two
+// distinct players, the same as two separate browsers would be.
 
 const PLAYER_ID_KEY = 'themeGuess.playerId';
 const PLAYER_NAME_KEY = 'themeGuess.playerName';
 
 /** Returns the persisted `playerId`, generating and storing a new
- * `crypto.randomUUID()` the first time this browser is seen. */
+ * `crypto.randomUUID()` the first time this tab is seen. */
 export function getOrCreatePlayerId(): string {
-  const existing = localStorage.getItem(PLAYER_ID_KEY);
+  const existing = sessionStorage.getItem(PLAYER_ID_KEY);
   if (existing) return existing;
   const created = crypto.randomUUID();
-  localStorage.setItem(PLAYER_ID_KEY, created);
+  sessionStorage.setItem(PLAYER_ID_KEY, created);
   return created;
 }
 
@@ -20,14 +23,14 @@ export function getOrCreatePlayerId(): string {
  * a generated default derived from the player's id so `identify` never
  * has to send an empty `name`. */
 export function getPlayerName(): string {
-  const existing = localStorage.getItem(PLAYER_NAME_KEY);
+  const existing = sessionStorage.getItem(PLAYER_NAME_KEY);
   if (existing) return existing;
   const fallback = `Player${getOrCreatePlayerId().slice(0, 4)}`;
-  localStorage.setItem(PLAYER_NAME_KEY, fallback);
+  sessionStorage.setItem(PLAYER_NAME_KEY, fallback);
   return fallback;
 }
 
 /** Overwrites the persisted display name. */
 export function setPlayerName(name: string): void {
-  localStorage.setItem(PLAYER_NAME_KEY, name);
+  sessionStorage.setItem(PLAYER_NAME_KEY, name);
 }