feat(phase-4): complete UX redesign — FileManagerView, FolderTreeItem, test suite, and all Phase 4 fixes

Adds the unified file manager view (Windows Explorer-style), collapsible
folder tree sidebar item, full vitest test suite (55 tests, 4 files), and
commits all Phase 4 backend/frontend fixes that were staged but uncommitted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.planning/config.json

@@ -9,7 +9,11 @@
     "plan_check": true,
     "verifier": true,
     "nyquist_validation": true,
-    "auto_advance": false
+    "auto_advance": false,
+    "test_gate": true,
+    "security_check": true,
+    "bugfix_max_lines": 50,
+    "require_root_cause_fix": true
   },
   "ship": {
     "pr_body_sections": [