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.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/web/src/i18n/en.ts

@@ -61,6 +61,54 @@ export default {
   dashboard: {
     title: 'Worktrees',
     allSessions: 'All sessions',
+    groups: 'Groups',
+  },
+  groups: {
+    title: 'Groups',
+    new: 'New group',
+    create: 'Create',
+    creating: 'Creating…',
+    nameLabel: 'Group name',
+    namePlaceholder: 'e.g. Payments stack',
+    colorLabel: 'Color',
+    reposLabel: 'Repositories',
+    empty: 'No group yet — create one above.',
+    noRepos: 'No repository selected.',
+    noReposInGroup: 'No repository in this group yet — add some below.',
+    noReposRegistered: 'No repository registered yet — add some from the dashboard first.',
+    open: 'Open',
+    edit: 'Edit repositories',
+    editDone: 'Done',
+    remove: 'Delete',
+    confirmDelete: 'Confirm delete',
+    repoCount: 'no repo | 1 repo | {n} repos',
+    dashboard: 'Dashboard',
+    missingRepo: 'Repository unavailable',
+    removeFromGroup: 'Remove from group',
+    sessionsActive: 'no active session | 1 active session | {n} active sessions',
+    viewList: 'Repos',
+    viewGrid: 'Terminals',
+    gridEmpty: 'No active session in this group.',
+    gridCapped: 'Showing {shown} of {total} sessions — close some terminals to see the rest.',
+    newFeature: 'New cross-repo feature',
+  },
+  crossRepo: {
+    title: 'New cross-repo feature',
+    intro: 'Create the same worktree across the selected repos, in one action.',
+    branchLabel: 'Branch name',
+    branchPlaceholder: 'feature/…',
+    newBranch: 'create branch',
+    baseRefLabel: 'Base ref (optional)',
+    startLabel: 'Start session',
+    startNone: 'no session',
+    reposLabel: 'Apply to',
+    create: 'Create across {n} repos',
+    creating: 'Creating…',
+    retryFailed: 'Retry failed',
+    close: 'Close',
+    pending: 'pending…',
+    ok: 'created',
+    error: 'failed',
   },
   repos: {
     add: 'Add repo',