refactor(presets): use --katex instead of --embed-resources --mathml

Problem: `--embed-resources` with `--mathml` caused pandoc to inline all
assets at compile time, adding ~15s per save for KaTeX-heavy documents.

Solution: Default to `--katex`, which inserts a CDN `<script>` tag and
defers rendering to the browser. Users can opt into `--embed-resources`
or `--mathml` via `extra_args`.

lua/preview/presets.lua

@@ -224,7 +224,7 @@ M.markdown = {
   ft = 'markdown',
   cmd = { 'pandoc' },
   args = function(ctx)
-    return { ctx.file, '-s', '--embed-resources', '--mathml', '-o', ctx.output }
+    return { ctx.file, '-s', '--katex', '-o', ctx.output }
   end,
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
@@ -249,8 +249,7 @@ M.github = {
       'gfm',
       ctx.file,
       '-s',
-      '--embed-resources',
-      '--mathml',
+      '--katex',
       '--css',
       'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
       '-o',

spec/presets_spec.lua

@@ -294,11 +294,11 @@ describe('presets', function()
       assert.are.same({ 'pandoc' }, presets.markdown.cmd)
     end)
 
-    it('returns args with standalone, embed-resources, and mathml flags', function()
+    it('returns args with standalone and katex flags', function()
       local args = presets.markdown.args(md_ctx)
       assert.is_table(args)
       assert.are.same(
-        { '/tmp/document.md', '-s', '--embed-resources', '--mathml', '-o', '/tmp/document.html' },
+        { '/tmp/document.md', '-s', '--katex', '-o', '/tmp/document.html' },
         args
       )
     end)
@@ -382,7 +382,7 @@ describe('presets', function()
       assert.are.same({ 'pandoc' }, presets.github.cmd)
     end)
 
-    it('returns args with standalone, embed-resources, mathml, and css flags', function()
+    it('returns args with standalone, katex, and css flags', function()
       local args = presets.github.args(md_ctx)
       assert.is_table(args)
       assert.are.same({
@@ -390,8 +390,7 @@ describe('presets', function()
         'gfm',
         '/tmp/document.md',
         '-s',
-        '--embed-resources',
-        '--mathml',
+        '--katex',
         '--css',
         'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
         '-o',