Save active sessions in browser local storage

Joining a game now records the session (game ID, player ID, plus
player/rat/crew names) in localStorage via the new lib/sessions.js, and
the dashboard keeps it enriched as state loads (so the rat name tracks
earned Names). The home page lists saved sessions in a "Rejoin a Crew"
menu, most-recent first, alongside creating a new game.

Deleting an entry only touches localStorage, never the backend, and is
undoable: the row greys out with an Undo button and is dropped from
storage immediately, so a page refresh finalizes the removal.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/Join.svelte

@@ -3,6 +3,7 @@
     import { push } from 'svelte-spa-router';
     import { apiRequest } from '../lib/api';
     import { setPageTitle } from '../lib/title';
+    import { saveSession } from '../lib/sessions';
     export let params = {};
 
     onMount(() => setPageTitle('Join the Crew'));
@@ -21,6 +22,9 @@
         error = '';
         try {
             const data = await apiRequest(`/game/${gameId}/join`, 'POST', { name: playerName });
+            // Remember this session so the home page can offer a rejoin link;
+            // the dashboard enriches it with crew/rat names once state loads.
+            saveSession({ gameId, playerId: data.id, playerName });
             push(`/game/${gameId}/player/${data.id}`);
         } catch (err) {
             error = err.message;