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-access-tui

@@ -47,6 +47,13 @@ REMOTE="${REMOTE:-user@vps.example.com}"
 read -r -p "Tunnel port on remote [2222]: " PORT
 PORT="${PORT:-2222}"
 
+# Ask for the remote folder that should be mounted (first positional of backtunnel-access)
+read -r -p "Remote folder path to mount [~/]: " FOLDER
+FOLDER="${FOLDER:-~/}"
+if [[ "$FOLDER" == "~" ]]; then
+  FOLDER="~/"
+fi
+
 read -r -p "Mount point [${DEFAULT_MP}]: " MP
 MP="${MP:-$DEFAULT_MP}"
 
@@ -62,7 +69,7 @@ if [[ ! -w "$MP" ]]; then
 fi
 
 echo
-echo "Running: backtunnel-access '<remote-folder>' from '$REMOTE' -p '$PORT' -m '$MP'"
-echo "Note: you'll be prompted on the remote for the exact folder (as per your workflow)."
+echo "Running: backtunnel-access '${FOLDER}' from '${REMOTE}' -p '${PORT}' -m '${MP}'"
+echo "Note: the folder is accessed via SFTP on the remote through the reverse tunnel."
 # Replace this process with backtunnel-access (no return to this script after exec)
-exec backtunnel-access "$MP" from "$REMOTE" -p "$PORT" -m "$MP"
+exec backtunnel-access "$FOLDER" from "$REMOTE" -p "$PORT" -m "$MP"