feat(p8): vue IDE /workspace (arbre + Monaco + diff + terminal)

- backend: garde-fou conflit d'édition par mtime (GET renvoie mtime, PUT refuse en 409 STALE_FILE si baseMtime périmé)
- lib: wt-key (base64url du chemin worktree), diff-parse (parseur pur du diff unifié → hunks)
- composables: useMonaco (import dynamique → chunk isolé), useWorkspaceLayout (largeurs/panneau persistés)
- components/workspace: GitStatusBadge, FileTree+FileTreeNode (lazy fs/list includeFiles), ChangedFilesPanel, DiffViewer (coloration +/-), MonacoEditor (Ctrl+S + bannière conflit reload/overwrite)
- views/WorkspaceView: 3 colonnes desktop redimensionnables + SegmentedControl mobile, header live (GitStatusBadge), watchWorktree → refresh diff/changements, terminal de la session corrélée
- router meta 'ide' + route /workspace/:repoId/:wt ; App.vue plein écran sans AppShell ; WorktreeCard bouton « Ouvrir l'IDE »
- deps: monaco-editor (lazy, vendor-monaco isolé, 0 impact bundle initial) ; vite manualChunks
- i18n EN+FR (workspace/editor/diff/git) ; tests diff-parse + wt-key ; acceptance-p8.mjs (mtime/409 + diff)

packages/web/src/components/WorktreeCard.vue

@@ -15,6 +15,9 @@
     <p class="truncate font-mono text-xs text-zinc-500" :title="worktree.path">{{ worktree.path }}</p>
 
     <div class="mt-1 flex flex-wrap items-center gap-2">
+      <RouterLink class="btn text-xs" :to="{ name: 'workspace', params: { repoId: worktree.repoId, wt: encodeWtKey(worktree.path) } }">
+        {{ t('workspace.open') }}
+      </RouterLink>
       <RouterLink
         v-for="s in visibleSessions"
         :key="s.id"
@@ -118,6 +121,7 @@ import type { SessionSummary, WorktreeSummary } from '@arboretum/shared';
 import { useWorktreesStore } from '../stores/worktrees';
 import { useSessionsStore } from '../stores/sessions';
 import { ApiError } from '../lib/api';
+import { encodeWtKey } from '../lib/wt-key';
 import SessionStateBadge from './SessionStateBadge.vue';
 import DialogPrompt from './DialogPrompt.vue';