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/STATE.md

@@ -3,14 +3,14 @@ gsd_state_version: 1.0
 milestone: v1.0
 milestone_name: milestone
 current_phase: 4
-status: planned
-last_updated: "2026-05-25T16:00:00.000Z"
+status: completed
+last_updated: "2026-05-28T14:59:51.958Z"
 progress:
   total_phases: 5
-  completed_phases: 3
-  total_plans: 33
-  completed_plans: 15
-  percent: 60
+  completed_phases: 4
+  total_plans: 24
+  completed_plans: 24
+  percent: 80
 ---
 
 # Project State