Align menuconfig key bindings and popup rendering

[jserv]

Key binding changes to match lxdialog/menubox.c:

• S sets YES (was save shortcut, no C equivalent)
• H/? shows help (was vi-style leave-menu)
• Remove vi navigation keys j/k/l/g/G (free for hotkey search)
• Remove Python-only shortcuts o (load) and d (save-minimal)
• Merge e/x/ESC/Backspace into single leave-menu handler
• Quit dialog uses 2 buttons (Yes/No) matching C dialog_yesno

Popup border visibility fix:

• Use body_style for all popup box edges (the C bold-white-on-white convention relies on ncurses ACS rendering that raw terminal output cannot replicate, making left/top borders invisible)
• Extract _draw_title, _draw_separator helpers to eliminate duplication
• Clamp text/button positions to protect border characters

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Summary by cubic

Aligns menuconfig key bindings with lxdialog and fixes popup rendering so borders are always visible. Removes vi-only keys, unifies exit behavior, and makes dialogs match the C UI.

• New Features

  • Match lxdialog keys: S sets YES; H/? opens help.
  • Remove vi navigation and Python-only save/load keys; unify ESC/Backspace/E/X behavior; quit dialog now uses Yes/No.

• Bug Fixes

  • Make popup borders visible by using body_style on all edges; add shared title and separator rendering.
  • Clamp text and button positions to avoid overwriting border characters.

^{Written for commit f1c3485. Summary will update on new commits.}