feat: mount under \$XDG_RUNTIME_DIR/sshfs/ instead of ~/Servers/

Aligns with XDG Base Directory spec: $XDG_RUNTIME_DIR is the defined
location for non-essential runtime files (ephemeral, user-owned,
session-scoped). sshfs mounts fit that definition exactly, and the
tmpfs backing means orphaned mountpoint dirs vanish on logout instead
of accumulating.

- verify_mount_dir reads $XDG_RUNTIME_DIR, falls back to
  /run/user/<uid>/ via os.Getuid().
- Existing path-traversal guard and symlink rejection carry over
  unchanged.
- Tests switched from t.Setenv("HOME") to t.Setenv("XDG_RUNTIME_DIR").

File-manager sidebar visibility is unaffected — gvfs surfaces FUSE
mounts via /proc/mounts regardless of mountpoint location.

main.go

@@ -183,11 +183,11 @@ func run_editor(mount string) error {
 }
 
 func verify_mount_dir(name string) (string, error) {
-	homedir, err := os.UserHomeDir()
-	if err != nil {
-		return "", fmt.Errorf("resolve home dir: %w", err)
+	runtime := os.Getenv("XDG_RUNTIME_DIR")
+	if runtime == "" {
+		runtime = filepath.Join("/run/user", strconv.Itoa(os.Getuid()))
 	}
-	base := filepath.Join(homedir, "Servers")
+	base := filepath.Join(runtime, "sshfs")
 	if err := os.MkdirAll(base, 0700); err != nil {
 		return "", fmt.Errorf("create base %q: %w", base, err)
 	}