feat: groupes de travail (P5)

Ajoute la notion de groupe — collection nommée de repos (membership légère
persistée, many-to-many) — pour piloter plusieurs repos en simultané :

- DB : migration #5 (tables groups + group_repos, FK ON DELETE CASCADE).
- GroupManager (synchrone, EventEmitter) + routes REST /api/v1/groups ;
  ne persiste que la membership, worktrees/sessions filtrés par repoId côté client.
- Protocole : GroupSummary, DTOs, topic WS « groups », events group_update/removed
  (additifs, PROTOCOL_VERSION inchangé).
- Web : store groups, GroupsListView, GroupView (réutilise RepoSection),
  grille multi-terminaux (TerminalGrid/TerminalCell), action « feature cross-repo »
  (orchestration client, tolérante aux échecs partiels), routes + i18n EN/FR.
- Tests : group-manager.test.ts + extension protocol.test.ts ; acceptance-p5.mjs.

packages/web/src/router/index.ts

@@ -8,6 +8,8 @@ export const router = createRouter({
     { path: '/', name: 'dashboard', component: () => import('../views/DashboardView.vue') },
     { path: '/sessions', name: 'sessions', component: () => import('../views/SessionsListView.vue') },
     { path: '/sessions/:id', name: 'session', component: () => import('../views/SessionView.vue') },
+    { path: '/groups', name: 'groups', component: () => import('../views/GroupsListView.vue') },
+    { path: '/groups/:id', name: 'group', component: () => import('../views/GroupView.vue') },
     { path: '/:pathMatch(.*)*', redirect: '/' },
   ],
 });