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-share

@@ -75,6 +75,13 @@
 
 set -euo pipefail
 
+# Host key checking policy: env BACKTUNNEL_HOSTKEY_POLICY = yes|no|ask|accept-new (default: accept-new)
+HKP="${BACKTUNNEL_HOSTKEY_POLICY:-accept-new}"
+case "$HKP" in
+  yes|no|ask|accept-new) ;;
+  *) HKP="accept-new" ;;
+esac
+
 # ----------------------------
 # Config discovery
 # Purpose: choose the highest-precedence profiles.ini available.
@@ -464,7 +471,7 @@ ${AUTH_CMD}
 ${INVITE_CMD}
 
 # Unmount when done:
-fusermount -u '${INVITE_MOUNT}' || fusermount3 -u '${INVITE_MOUNT}'
+backtunnel-umount '${INVITE_MOUNT}'
 EOT
     )
   else
@@ -476,7 +483,7 @@ EOT
 ${INVITE_CMD}
 
 # Unmount when done:
-fusermount -u '${INVITE_MOUNT}' || fusermount3 -u '${INVITE_MOUNT}'
+backtunnel-umount '${INVITE_MOUNT}'
 EOT
     )
   fi
@@ -512,7 +519,7 @@ echo "To stop sharing early: press Ctrl+C in this window."
 # Pre-flight: warn if remote loopback port already in use (best-effort)
 # Purpose: give an actionable warning before attempting the -R bind.
 # ----------------------------
-if ssh -o BatchMode=yes -o ConnectTimeout=5 "${REMOTE_USER}@${REMOTE_HOST}" \
+if ssh -o BatchMode=yes -o StrictHostKeyChecking="$HKP" -o ConnectTimeout=5 "${REMOTE_USER}@${REMOTE_HOST}" \
      "command -v nc >/dev/null 2>&1 && nc -z 127.0.0.1 ${TUNNEL_PORT}"; then
   echo "⚠️  Port ${TUNNEL_PORT} on remote 127.0.0.1 appears in use; choose another with -p." >&2
   # You may 'exit 1' here if you prefer a hard failure
@@ -545,6 +552,7 @@ ssh -N \
   -o ExitOnForwardFailure=yes \
   -o ServerAliveInterval=15 \
   -o ServerAliveCountMax=3 \
+  -o StrictHostKeyChecking="$HKP" \
   -R "${TUNNEL_PORT}:localhost:${LOCAL_SSH_PORT}" \
   -- "${REMOTE_USER}@${REMOTE_HOST}" &
 SSH_PID=$!