fix(mappings): add global entry-point keymaps and correct <Leader>ww/<Leader>wt targets

Two bugs prevented keymaps from working after a Dein install:

1. <Leader>ww and <Leader>wt both called diary_today() instead of
   wiki_index() / wiki_tab_index(). Fixed in keymaps.lua (Neovim)
   and ftplugin/vimwiki.vim (Vim).

2. All keymaps were buffer-local (<buffer> / buffer=bufnr), so they
   only activated inside an already-open .wiki file. Users had no way
   to reach the wiki from any other buffer, making the plugin appear
   broken on a fresh Vim start.

   Fix: register the eight <Leader>w* entry-point mappings globally —
   in setup() for Neovim (lua/nuwiki/init.lua) and in plugin/nuwiki.vim
   for plain Vim. To avoid a chicken-and-egg LSP dependency, the global
   mappings open files directly from config (wiki_root, diary_rel_path,
   file_extension); the LSP auto-starts via the FileType autocmd once
   the vimwiki buffer loads. Three new autoload helpers
   (open_wiki_path, open_diary_path, open_diary_index_path) provide the
   LSP-free path for the Vim side.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -115,8 +115,8 @@ if !has('nvim')
 
   if !get(g:, 'nuwiki_no_default_mappings', 0)
     " Wiki prefix
-    nnoremap <silent><buffer> <Leader>ww  :call nuwiki#commands#diary_today()<CR>
-    nnoremap <silent><buffer> <Leader>wt  :call nuwiki#commands#diary_today()<CR>
+    nnoremap <silent><buffer> <Leader>ww  :call nuwiki#commands#wiki_index(0)<CR>
+    nnoremap <silent><buffer> <Leader>wt  :call nuwiki#commands#wiki_tab_index(0)<CR>
     nnoremap <silent><buffer> <Leader>ws  :call nuwiki#commands#wiki_ui_select()<CR>
     nnoremap <silent><buffer> <Leader>wi  :call nuwiki#commands#diary_index()<CR>
     nnoremap <silent><buffer> <Leader>w<Leader>w  :call nuwiki#commands#diary_today()<CR>