feat: rename watch → toggle, auto-compile on start, built-in opener

Problem: :Preview watch only registered a BufWritePost autocmd without
compiling immediately, required boilerplate to open output files after
first compilation, and was misleadingly named.

Solution: Rename watch → toggle throughout. M.toggle now compiles
immediately on activation. Add an open field to ProviderConfig: true
calls vim.ui.open(), a string[] runs the command with the output path
appended, tracked per-buffer so the file opens only once. All presets
default to { 'xdg-open' }. Health check validates opener binaries.
Guard the async compile callback against invalid buffer ids.

spec/helpers.lua

@@ -20,8 +20,11 @@ function M.delete_buffer(bufnr)
 end
 
 function M.reset_config(opts)
-  vim.g.preview = opts
-  require('preview')._test.reset()
+  local preview = require('preview')
+  preview._test.reset()
+  if opts then
+    preview.setup(opts)
+  end
 end
 
 return M