fix(compiler): check executable before spawning process

Problem: if a configured binary was missing or not in PATH, vim.system
would fail silently or with a cryptic OS error. The user had no
actionable feedback without running :checkhealth.

Solution: check vim.fn.executable() at the start of M.compile() and
notify with an ERROR-level message pointing to :checkhealth preview
if the binary is not found.

lua/preview/compiler.lua

@@ -58,6 +58,14 @@ end
 function M.compile(bufnr, name, provider, ctx, opts)
   opts = opts or {}
 
+  if vim.fn.executable(provider.cmd[1]) ~= 1 then
+    vim.notify(
+      '[preview.nvim]: "' .. provider.cmd[1] .. '" is not executable (run :checkhealth preview)',
+      vim.log.levels.ERROR
+    )
+    return
+  end
+
   if vim.bo[bufnr].modified then
     vim.cmd('silent! update')
   end