Filesystem Desktop Extension: path validation fails on Windows with spaces and German umlauts in subdirectory names

The Filesystem Desktop Extension (installed via Claude Desktop's built-in extension
directory) fails to access subdirectories that contain spaces or German umlauts
(ä, ö, ü, Ä, Ö, Ü) in their names. The root drive letter is accepted, but all
subdirectory access is blocked with a path validation error.

This is a significant issue for enterprise environments in German-speaking countries,
where folder names like 03- FORMBLÄTTER or Qualitätsmanagement are standard on
corporate file servers.

I'm aware of #447 and #2437, and that PR #543 added support for spaces in the npx-based
server. However, the issue persists in the Claude Desktop Filesystem Extension
(installed via Settings → Extensions → Filesystem), suggesting the extension may bundle
an older version without the fix.

To Reproduce

1. Install the Filesystem Desktop Extension via Claude Desktop → Settings → Extensions
   → Filesystem
2. Add a mapped network drive (e.g. Q:\) as an allowed directory
3. The network drive's UNC path has no spaces (e.g. \\server\gpe-qm)
4. The drive contains subdirectories with spaces and/or umlauts, e.g.:
   • Q:\03- FORMBLÄTTER
   • Q:\Qualitätsmanagement\Dokumente
5. Ask Claude to list or read files in any subdirectory

Expected behavior

Claude should be able to traverse and read files in subdirectories regardless of
spaces or special characters (umlauts) in folder names.

Actual behavior

Claude reports:

"Der Filesystem-Server verweigert aktuell den Zugriff auf alle Unterordner
(vermutlich ein Pfad-Validierungsfehler bei Leerzeichen in UNC-Pfaden).
Ich kann daher keine Dateiinhalte lesen."

The root drive Q:\ is recognized as an allowed directory, but access to ALL
subdirectories fails — not just those with spaces/umlauts.

Environment

• OS: Windows 10/11 (Enterprise, managed via Microsoft Intune)
• Claude Desktop: Latest version (March 2026)
• Filesystem Extension: Installed from built-in extension directory
• Drive mapping: Q: → \\server\gpe-qm (UNC path has no spaces)
• Subdirectory examples: Q:\03- FORMBLÄTTER, contains spaces + umlaut (Ä)

Additional context

• The UNC path behind the drive letter contains no spaces
  (\\srvgpenwm3\gpe-qm) — confirmed via net use Q:
• The issue affects the Desktop Extension variant, not a manually configured
  npx-based MCP server
• Related issues: #447, #2437
• Related fix: PR #543 (merged Aug 2025) — may not be included in the Desktop
  Extension build

Compare the Claude Desktop Filesystem Extension build with the npx-based server and PR #543, which the issue identifies as the related path-validation fix. Reproduce on Windows with mapped Q: paths containing spaces and umlauts, then verify the extension can traverse and read those subdirectories without rejecting valid paths.