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/i18n/en.ts

@@ -202,6 +202,39 @@ export default {
     create: 'Create',
     creating: 'Creating…',
   },
+  workspace: {
+    open: 'Open IDE',
+    files: 'Files',
+    changes: 'Changes',
+    noChanges: 'No changes',
+    changesTruncated: 'Too many changes — list truncated.',
+    terminal: 'Terminal',
+    editor: 'Editor',
+    diff: 'Diff',
+    noFileOpen: 'No file open',
+    noFileHint: 'Pick a file in the tree to edit it, or a changed file to view its diff.',
+    noSession: 'No session here',
+    noSessionHint: 'Start a session on this worktree from the dashboard to get a terminal.',
+  },
+  editor: {
+    save: 'Save',
+    unsaved: 'Unsaved changes',
+    conflict: 'This file changed on disk since you opened it.',
+    reload: 'Reload',
+    overwrite: 'Overwrite',
+  },
+  diff: {
+    binary: 'Binary file — no text diff.',
+    tooLarge: 'Diff too large to display.',
+    noChanges: 'No changes in this file.',
+  },
+  git: {
+    ahead: 'commits ahead',
+    behind: 'commits behind',
+    staged: 'staged',
+    unstaged: 'unstaged',
+    conflicts: 'conflicts',
+  },
   terminal: {
     observer: 'observer (read-only)',
     sessionEnded: 'Session ended',