Add backtunnel-umount script, host key policy support, and extend TUI/CLI integration

Introduce `backtunnel-umount` as a portable unmount helper, preferring `fusermount3`, `fusermount`, or `umount`. Add `BACKTUNNEL_HOSTKEY_POLICY` for configurable host key handling in `backtunnel-share` and `backtunnel-access`. Update TUIs for remote folder prompts and mount point handling. Enhance bash completion for TUI commands with directory suggestions. Revamp terminal selection logic in `backtunnel-open-term` to prioritize modern emulators like wezterm. Extend tests with scaffolds for host key policy and unmount behavior. Update README with new scripts, workflows, features, and troubleshooting tips.

scripts/backtunnel-umount

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-3.0-or-later
+# Name: backtunnel-umount
+# Summary: Unmount a BackTunnel FUSE mount point using the best available helper.
+# Usage:
+#   backtunnel-umount <mountpoint>
+# Notes:
+#   - Prefers fusermount3, then fusermount; falls back to umount.
+#   - Expands a leading "~" in the mountpoint.
+set -euo pipefail
+
+usage() {
+  echo "Usage: $(basename "$0") <mountpoint>" >&2
+  exit 1
+}
+
+case "${1:-}" in
+  -h|--help) usage ;;
+esac
+
+MP="${1:-}"
+[[ -n "$MP" ]] || usage
+
+# Expand leading ~
+if [[ "$MP" == "~"* ]]; then
+  MP="${MP/#\~/$HOME}"
+fi
+
+if command -v fusermount3 >/dev/null 2>&1; then
+  exec fusermount3 -u -- "$MP"
+elif command -v fusermount >/dev/null 2>&1; then
+  exec fusermount -u -- "$MP"
+else
+  exec umount -- "$MP"
+fi