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.

completions/backtunnel.bash

@@ -241,4 +241,30 @@ _backtunnel_complete() {
 
 # Register for both commands
 complete -F _backtunnel_complete backtunnel-share
-complete -F _backtunnel_complete backtunnel-access
+complete -F _backtunnel_complete backtunnel-access
+
+# Minimal TUI completers: complete only the first positional as a directory
+_backtunnel_access_tui_complete() {
+  local cur
+  cur="${COMP_WORDS[COMP_CWORD]}"
+  if [[ ${COMP_CWORD} -eq 1 ]]; then
+    compopt -o filenames 2>/dev/null
+    mapfile -t COMPREPLY < <(compgen -d -- "$cur")
+  else
+    COMPREPLY=()
+  fi
+}
+_backtunnel_share_tui_complete() {
+  local cur
+  cur="${COMP_WORDS[COMP_CWORD]}"
+  if [[ ${COMP_CWORD} -eq 1 ]]; then
+    compopt -o filenames 2>/dev/null
+    mapfile -t COMPREPLY < <(compgen -d -- "$cur")
+  else
+    COMPREPLY=()
+  fi
+}
+
+# Register TUI completion
+complete -F _backtunnel_access_tui_complete backtunnel-access-tui
+complete -F _backtunnel_share_tui_complete backtunnel-share-tui