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.

README.md

@@ -1,6 +1,6 @@
 Quickly mount remote systems via SSHFS based on your ssh_config
 
-Static mount dir is currently `~/Servers/<Host>` (uses the ssh_config alias as label, not the resolved IP).
+Mounts land under `$XDG_RUNTIME_DIR/sshfs/<Host>` (typically `/run/user/$UID/sshfs/<Host>`), using the ssh_config alias as label, not the resolved IP. The directory is auto-cleaned on logout (tmpfs).
 
 # Install