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.

lua/preview/health.lua

@@ -25,6 +25,14 @@ function M.check()
     else
       vim.health.error('filetype "' .. ft .. '": ' .. bin .. ' not found')
     end
+    if type(provider.open) == 'table' then
+      local opener = provider.open[1]
+      if vim.fn.executable(opener) == 1 then
+        vim.health.ok('filetype "' .. ft .. '": opener ' .. opener .. ' found')
+      else
+        vim.health.error('filetype "' .. ft .. '": opener ' .. opener .. ' not found')
+      end
+    end
   end
 end