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/presets.lua

@@ -2,6 +2,7 @@ local M = {}
 
 ---@type preview.ProviderConfig
 M.typst = {
+  ft = 'typst',
   cmd = { 'typst', 'compile' },
   args = function(ctx)
     return { ctx.file }
@@ -9,10 +10,12 @@ M.typst = {
   output = function(ctx)
     return ctx.file:gsub('%.typ$', '.pdf')
   end,
+  open = { 'xdg-open' },
 }
 
 ---@type preview.ProviderConfig
 M.latex = {
+  ft = 'tex',
   cmd = { 'latexmk' },
   args = function(ctx)
     return { '-pdf', '-interaction=nonstopmode', ctx.file }
@@ -23,18 +26,49 @@ M.latex = {
   clean = function(ctx)
     return { 'latexmk', '-c', ctx.file }
   end,
+  open = { 'xdg-open' },
 }
 
 ---@type preview.ProviderConfig
 M.markdown = {
+  ft = 'markdown',
   cmd = { 'pandoc' },
   args = function(ctx)
-    local output = ctx.file:gsub('%.md$', '.pdf')
-    return { ctx.file, '-o', output }
+    local output = ctx.file:gsub('%.md$', '.html')
+    return { ctx.file, '-s', '--embed-resources', '-o', output }
   end,
   output = function(ctx)
-    return ctx.file:gsub('%.md$', '.pdf')
+    return ctx.file:gsub('%.md$', '.html')
   end,
+  clean = function(ctx)
+    return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
+  end,
+  open = { 'xdg-open' },
+}
+
+---@type preview.ProviderConfig
+M.github = {
+  ft = 'markdown',
+  cmd = { 'pandoc' },
+  args = function(ctx)
+    local output = ctx.file:gsub('%.md$', '.html')
+    return {
+      ctx.file,
+      '-s',
+      '--embed-resources',
+      '--css',
+      'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/github.css',
+      '-o',
+      output,
+    }
+  end,
+  output = function(ctx)
+    return ctx.file:gsub('%.md$', '.html')
+  end,
+  clean = function(ctx)
+    return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
+  end,
+  open = { 'xdg-open' },
 }
 
 return M