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-open-term

@@ -78,7 +78,8 @@ cmd=( "$@" )
     if [[ -n "${KDE_FULL_SESSION:-}" ]] && command -v konsole >/dev/null 2>&1; then
       echo "konsole"; return
     fi
-    for t in kitty alacritty kgx gnome-terminal tilix xfce4-terminal konsole xterm; do
+    # Prefer widely used modern terminals first when not on KDE
+    for t in wezterm kitty alacritty kgx gnome-terminal tilix xfce4-terminal konsole xterm; do
       if command -v "$t" >/dev/null 2>&1; then echo "$t"; return; fi
     done
     echo ""  # none
@@ -101,6 +102,7 @@ cmd=( "$@" )
   # Run command in terminal (use hold/noclose if supported)
   case "$term" in
     konsole)         exec konsole --noclose -e "${cmd[@]}" ;;
+    wezterm)         exec wezterm start -- "${cmd[@]}" ;;
     kitty)           exec kitty -e "${cmd[@]}" ;;
     alacritty)       exec alacritty -e "${cmd[@]}" ;;
     gnome-terminal)  exec gnome-terminal -- bash -lc "exec $shell_cmd" ;;