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/services/auth.py

@@ -356,7 +356,7 @@ async def check_hibp(password: str) -> bool:
     Returns True if the password has been breached, False otherwise.
     On network error: logs a warning and returns False (fail-open, T-02-06).
     """
-    sha1 = hashlib.sha1(password.encode("utf-8")).hexdigest().upper()
+    sha1 = hashlib.sha1(password.encode("utf-8"), usedforsecurity=False).hexdigest().upper()  # noqa: S324 — HIBP k-anonymity protocol mandates SHA-1; not used as a security primitive
     prefix, suffix = sha1[:5], sha1[5:]
 
     try: