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/test/wt-key.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from 'vitest';
+import { encodeWtKey, decodeWtKey } from '../src/lib/wt-key';
+
+describe('wt-key (base64url)', () => {
+  const paths = [
+    '/home/johan/WebstormProjects/arboretum',
+    '/srv/code/feature-branch',
+    '/tmp/with space/and-dash',
+    '/projets/accentué/café/é',
+    '/a/b/c/d/e/f/g',
+  ];
+
+  it('round-trip encode → decode pour divers chemins', () => {
+    for (const p of paths) expect(decodeWtKey(encodeWtKey(p))).toBe(p);
+  });
+
+  it('produit un segment d’URL sûr (pas de + / =)', () => {
+    for (const p of paths) {
+      const key = encodeWtKey(p);
+      expect(key).not.toMatch(/[+/=]/);
+      expect(encodeURIComponent(key)).toBe(key); // déjà URL-safe
+    }
+  });
+});