fix(commands)+docs: even out :NuwikiUISelect; exhaustive subgroup key lists

- ftplugin/vimwiki.vim: add buffer-local :NuwikiUISelect to the Vim path
  (it previously existed only on the Neovim path, contradicting the docs'
  "buffer-local on every .wiki buffer" claim — a global fallback masked it).
  Now symmetric with :VimwikiUISelect.

- Make every keymap subgroup description list all of its bindings instead
  of trailing with "…": the README mappings-subgroups table, the README
  config-example comments, the README g:nuwiki_no_<group>_mappings rows,
  the doc/nuwiki.txt group→keys list, and the lua/nuwiki/config.lua comment.
  Also fixes the doc list that had dropped aH/iH from text_objects and adds
  the mouse group. Each notes that the <Leader>w prefix follows map_prefix.

helptags validates (no dup tags); all keymap suites pass (266+18+21 Vim,
275 Lua); :NuwikiUISelect confirmed defined on a Vim .wiki buffer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -121,6 +121,7 @@ if !has('nvim')
   " :Nuwiki* canonical aliases — Vim side.
   command! -buffer -count NuwikiIndex      call nuwiki#commands#wiki_index(<count>)
   command! -buffer -count NuwikiTabIndex   call nuwiki#commands#wiki_tab_index(<count>)
+  command! -buffer        NuwikiUISelect   call nuwiki#commands#wiki_ui_select()
   command! -buffer        NuwikiDiaryIndex call nuwiki#commands#diary_index()
   command! -buffer        NuwikiDiaryToday     call nuwiki#commands#diary_today()
   command! -buffer        NuwikiDiaryYesterday call nuwiki#commands#diary_yesterday()