fix(desktop): mirror Windows path identity in live overlay + WSL spelling

Addresses @teknium1's review of #61950:

- The desktop live overlay (workspace-groups.ts) matched cwd membership
  case-sensitively, so a fresh mixed-case/separator Windows session missed
  its explicit/auto project until the next backend tree refresh. Mirror the
  backend identity (isWindowsPath/comparisonSegments/pathKey) in isPathUnder,
  liveSessionProjectId, and overlayRepoLanes lane matching. Comparison-only —
  emitted ids/labels keep their spelling. POSIX stays case-sensitive.
- Backend _is_windows_path missed root-relative `\wsl.localhost\...` (single
  leading backslash), leaving that historical spelling case-sensitive. Classify
  any backslash-rooted path as Windows.

Tests: WSL-spelling collapse + explicit-project precedence (project_tree),
Windows/WSL live-overlay membership + POSIX case-sensitivity (workspace-groups).

tui_gateway/project_tree.py

@@ -62,7 +62,10 @@ def _segments(path: str) -> list[str]:
 
 def _is_windows_path(path: str) -> bool:
     value = (path or "").strip()
-    return bool(re.match(r"^[A-Za-z]:[/\\]", value)) or value.startswith(("\\\\", "//"))
+    # Drive-letter (`C:\…`), UNC (`\\srv`, `//srv`), or any backslash-rooted path
+    # — the root-relative `\wsl.localhost\…` / `\Users\…` spellings included. A
+    # single leading `/` stays POSIX (case-sensitive).
+    return bool(re.match(r"^[A-Za-z]:[/\\]", value)) or value.startswith(("\\", "//"))
 
 
 def _comparison_segments(path: str) -> list[str]: