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>

lua/nuwiki/install.lua

@@ -66,6 +66,28 @@ local function build_from_source(dest)
   return vim.fn.executable(dest) == 1
 end
 
+-- Gitea API base for this repo. Gitea has no GitHub-style
+-- `/releases/latest/download/<asset>` shortcut, so resolve the asset's real
+-- download URL (`browser_download_url`) through the API instead.
+local REPO_API = 'https://code.gfran.co/api/v1/repos/gffranco/nuwiki'
+
+local function latest_asset_url(asset_name)
+  local body = vim.fn.system({ 'curl', '-fsSL', REPO_API .. '/releases/latest' })
+  if vim.v.shell_error ~= 0 then
+    return nil
+  end
+  local ok, release = pcall(vim.json.decode, body)
+  if not ok or type(release) ~= 'table' or type(release.assets) ~= 'table' then
+    return nil
+  end
+  for _, a in ipairs(release.assets) do
+    if a.name == asset_name then
+      return a.browser_download_url
+    end
+  end
+  return nil
+end
+
 local function download_release(dest)
   local target = target_triple()
   if not target then
@@ -74,10 +96,10 @@ local function download_release(dest)
   if vim.fn.executable('curl') ~= 1 or vim.fn.executable('tar') ~= 1 then
     return false
   end
-  local url = string.format(
-    'https://code.gfran.co/gffranco/nuwiki/releases/latest/download/nuwiki-ls-%s.tar.gz',
-    target
-  )
+  local url = latest_asset_url('nuwiki-ls-' .. target .. '.tar.gz')
+  if not url then
+    return false
+  end
   local archive = vim.fn.tempname() .. '.tar.gz'
   vim.fn.system({ 'curl', '-fsSL', '-o', archive, url })
   if vim.v.shell_error ~= 0 then