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>

backend/tests/test_lmstudio.py

@@ -30,7 +30,7 @@ async def test_lmstudio_health_check():
 @pytest.mark.asyncio
 async def test_lmstudio_classify():
     from ai.lmstudio_provider import LMStudioProvider
-    from config import DEFAULT_SYSTEM_PROMPT
+    from services.classifier import _DEFAULT_SYSTEM_PROMPT
 
     provider = LMStudioProvider(
         base_url="http://host.docker.internal:1234",
@@ -39,7 +39,7 @@ async def test_lmstudio_classify():
     result = await provider.classify(
         document_text="This document is an invoice for software development services.",
         existing_topics=["Finance", "Legal", "HR"],
-        system_prompt=DEFAULT_SYSTEM_PROMPT,
+        system_prompt=_DEFAULT_SYSTEM_PROMPT,
     )
     # Result should have some topics assigned or suggested
     assert isinstance(result.topics, list)