feat: honor the daemon's [keys] table, prefix chords included

The daemon's config.toml [keys] table now drives GUI shortcuts too, so a
TUI habit such as prefix+v or alt+1..9 works in both clients (#162).

- keymap/daemon.rs reads [keys] leniently: Herdr owns and validates that
  file, so defaults apply when it is absent and entries the GUI cannot
  express (hyper, unmodified direct keys, actions without a GUI command)
  are skipped rather than failing the GUI config.
- Keymap layers catalog defaults, then daemon keys, then the GUI's own
  [keybindings]. A command named in [keybindings] keeps exactly the keys
  listed there; a GUI keystroke outranks the daemon's, even the prefix.
- Prefix mode follows the TUI rather than GPUI multi-stroke bindings: a
  keystroke interceptor arms on the prefix, runs the next key's chord or
  swallows it, passes a doubled prefix through, and cancels on Escape. A
  chord typed in a menu's text field closes the menu and runs.
- The status bar shows the armed prefix, the shortcut page renders
  chords, and the config watcher also reloads on daemon config changes.