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/workspace/GitStatusBadge.vue

@@ -0,0 +1,27 @@
+<template>
+  <span class="flex items-center gap-2 text-xs">
+    <span class="flex items-center gap-1 font-mono text-zinc-300">
+      <GitBranch :size="13" />{{ branch ?? t('worktrees.detached') }}
+    </span>
+    <span v-if="git.ahead" class="text-emerald-400" :title="t('git.ahead')">↑{{ git.ahead }}</span>
+    <span v-if="git.behind" class="text-amber-400" :title="t('git.behind')">↓{{ git.behind }}</span>
+    <span v-if="git.stagedCount" class="text-emerald-400" :title="t('git.staged')">●{{ git.stagedCount }}</span>
+    <span v-if="git.unstagedCount" class="text-amber-400" :title="t('git.unstaged')">○{{ git.unstagedCount }}</span>
+    <span v-if="git.conflictCount" class="text-rose-400" :title="t('git.conflicts')">⚠{{ git.conflictCount }}</span>
+    <span v-if="isClean" class="text-zinc-600">{{ t('worktrees.clean') }}</span>
+  </span>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue';
+import { useI18n } from 'vue-i18n';
+import { GitBranch } from '@lucide/vue';
+import type { WorktreeGitStatus } from '@arboretum/shared';
+
+const props = defineProps<{ git: WorktreeGitStatus; branch: string | null }>();
+const { t } = useI18n();
+
+const isClean = computed(
+  () => !props.git.dirtyCount && !props.git.ahead && !props.git.behind && !props.git.stagedCount && !props.git.unstagedCount,
+);
+</script>