fix(install): resolve release asset via Gitea API (download was always 404)

:NuwikiInstall always fell back to building from source because the
download URL used the GitHub-only shape
`/releases/latest/download/<asset>`, which this Gitea instance serves as
404. The release assets exist and are correctly named; only the URL was
wrong.

Resolve the asset's real download URL through the Gitea API instead: GET
`/api/v1/repos/.../releases/latest`, find the asset matching
`nuwiki-ls-<target>.tar.gz`, and download its `browser_download_url`
(`/releases/download/<tag>/<asset>`, which returns 200). Fixed in both
install paths — lua/nuwiki/install.lua (vim.json.decode) and
scripts/download_bin.vim (json_decode). Verified end to end: resolve →
download → extract → executable binary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/download_bin.vim

@@ -48,6 +48,30 @@ function! s:build_from_source() abort
   return executable(s:bin)
 endfunction
 
+" Gitea has no GitHub-style /releases/latest/download/<asset> shortcut, so
+" resolve the asset's real download URL (browser_download_url) via the API.
+function! s:latest_asset_url(asset) abort
+  let l:api = 'https://code.gfran.co/api/v1/repos/gffranco/nuwiki/releases/latest'
+  let l:body = system('curl -fsSL ' . shellescape(l:api))
+  if v:shell_error != 0
+    return ''
+  endif
+  try
+    let l:release = json_decode(l:body)
+  catch
+    return ''
+  endtry
+  if type(l:release) != type({}) || !has_key(l:release, 'assets')
+    return ''
+  endif
+  for l:item in l:release.assets
+    if get(l:item, 'name', '') ==# a:asset
+      return get(l:item, 'browser_download_url', '')
+    endif
+  endfor
+  return ''
+endfunction
+
 function! s:download_release() abort
   let l:target = s:target()
   if empty(l:target)
@@ -56,8 +80,10 @@ function! s:download_release() abort
   if !executable('curl') || !executable('tar')
     return 0
   endif
-  let l:url = 'https://code.gfran.co/gffranco/nuwiki/releases/latest/download/nuwiki-ls-'
-        \ . l:target . '.tar.gz'
+  let l:url = s:latest_asset_url('nuwiki-ls-' . l:target . '.tar.gz')
+  if empty(l:url)
+    return 0
+  endif
   let l:archive = tempname() . '.tar.gz'
   call system('curl -fsSL -o ' . shellescape(l:archive) . ' ' . shellescape(l:url))
   if v:shell_error != 0