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/phases/03-document-migration-multi-user-isolation/03-UAT.md

@@ -1,14 +1,15 @@
 ---
-status: testing
+status: complete
 phase: 03-document-migration-multi-user-isolation
 source: 03-01-SUMMARY.md, 03-02-SUMMARY.md, 03-03-SUMMARY.md, 03-04-SUMMARY.md, 03-05-SUMMARY.md
 started: 2026-05-24T00:00:00Z
-updated: 2026-05-24T00:00:00Z
+updated: 2026-05-25T00:00:00Z
+completed: 2026-05-25T00:00:00Z
 ---
 
 ## Current Test
 
-UAT-3 — QuotaBar visual in sidebar (needs browser confirmation)
+All tests complete — Phase 3 UAT passed.
 
 ## Tests
 
@@ -23,7 +24,8 @@ reported: "User confirmed upload works after fixes."
 
 ### 3. QuotaBar displays in sidebar
 expected: After the upload completes, look at the left sidebar. A quota bar widget is visible below the navigation links. It shows used/total storage (e.g. "1.2 MB / 100 MB") with an indigo-colored fill bar. No error state or broken layout.
-result: [pending]
+result: pass
+reported: "QuotaBar visible in sidebar with indigo fill bar. Confirmed by user 2026-05-25."
 
 ### 4. Quota rejection error block
 expected: Upload a file that would push usage over the user's quota limit (create a user via admin with a very small quota, e.g. 1 byte, or use an account already near-full). The upload row shows a red "Not enough storage" error block with role="alert", showing the rejected file size, current used bytes, and quota limit. A "Manage storage →" link appears. The quota bar does NOT increase past the limit.
@@ -62,9 +64,9 @@ reported: "document_tasks.py _run() resolves ai_provider from user.ai_provider w
 ## Summary
 
 total: 10
-passed: 9
+passed: 10
 issues: 0
-pending: 1
+pending: 0
 skipped: 0
 blocked: 0