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/components/TerminalCell.vue

@@ -0,0 +1,39 @@
+<template>
+  <!-- focus-within → anneau visuel : montre quelle cellule reçoit la frappe clavier -->
+  <div class="flex min-h-0 flex-col overflow-hidden rounded-lg border border-zinc-800 bg-[#09090b] focus-within:ring-2 focus-within:ring-sky-600">
+    <header class="flex items-center gap-2 border-b border-zinc-800 px-2 py-1">
+      <SessionStateBadge :session="session" />
+      <span class="truncate font-mono text-xs text-zinc-300" :title="session.cwd">{{ title }}</span>
+      <span class="ml-auto shrink-0 font-mono text-[11px] text-zinc-500">{{ session.command }}</span>
+    </header>
+    <DialogPrompt v-if="isWaiting" :session="session" class="px-2 pt-2" />
+    <div class="min-h-0 flex-1">
+      <TerminalView :session-id="session.id" mode="interactive" />
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue';
+import type { SessionSummary } from '@arboretum/shared';
+import { useWorktreesStore } from '../stores/worktrees';
+import SessionStateBadge from './SessionStateBadge.vue';
+import DialogPrompt from './DialogPrompt.vue';
+import TerminalView from './TerminalView.vue';
+
+const props = defineProps<{ session: SessionSummary }>();
+const worktrees = useWorktreesStore();
+
+// libellé : « repo · branche » si le worktree est connu, sinon le dernier segment du cwd.
+const title = computed(() => {
+  const wt = worktrees.worktrees.find((w) => w.path === props.session.cwd);
+  if (wt) {
+    const repo = worktrees.repos.find((r) => r.id === wt.repoId);
+    const branch = wt.branch ?? wt.head.slice(0, 7);
+    return repo ? `${repo.label} · ${branch}` : branch;
+  }
+  return props.session.cwd.split('/').filter(Boolean).pop() ?? props.session.cwd;
+});
+
+const isWaiting = computed(() => props.session.live && props.session.activity === 'waiting' && !!props.session.dialog);
+</script>